Well, imo, one of the biggest costs is accommodation. If I had no kids, dh and I could manage in a one bedroom apartment (we probably wouldn't, but we could), and our rent would probably be about half what we're currently paying. If you're looking at buying a house, then you're paying whatever your mortgage costs, regardless of whether you have just your dd, have another one, or whatever.
I live in a high COL area, and I really suck at meal planning right now. We're paying about $1200/month on our grocery bill. That includes all food, except take-out and restaurants. (we consider those to be entertainment expenses). It also includes dish soap, laundry detergent, diapers (dd2 isn't in cloth, although my others all were), dental hygiene supplies, soap, shampoo, etc. and most supplements that I buy (ie. everything that I get at the grocery store). I couldn't begin to figure out how much we'd be paying if it were just us, though. We're feeding an extra adult (not legally, but in terms of physical development, definitely), and he costs a bit...he's not small, and he's very active, plus three kids, although the toddler isn't eating much yet...
Other costs. Let's see - we pay $88-$110 (depending how many Tuesdays there are) a month for dd1's piano lessons, and another $48-$60 (again, depending how many classes) per month for ballet. DS2's dance class is only $24-$30, though...and his Tae Kwon Do is only $6.00/lesson. These classes mostly run during the school year. We probably pay about $1500 for dd1's regular extracurriculars and $500/$600 a year for ds2's regular classes. Memberships (Aquarium, Science World and the local farm) are about another $400/year. Then, there are occasional things, such as classes at the Ecology Centre, or behind-the-scenes trips to the farm. Most of those activities are around $15.00 per child. These are all quite normal, but are obviously not required. That kind of thing will vary a lot from one family to another.
We don't travel a lot, so I'm not sure what a family vacation would cost. You might want to research the kind of vacation you want a little, and factor that in.
Clothes are all over the map. I usually buy my kids a new jacket about once every year and half or so - depends how fast they outgrow them. We buy new shoes, socks and underwear, as needed. We don't buy a lot of day-to-day clothes, because my MIL gives the kids clothes as gifts on their birthdays, Christmas, "back-to-school" (we homeschool, but they still send new September clothes), and sometimes Easter. I'd say we probably spend somewhere around $150/year on clothes. I'm sure it's not much more than that, but I don't know the exact number.
I don't know what our medical/dental coverage cost, because dh gets them as benefits at work, so we only pay the tax. I've looked at the numbers, but I don't remember the totals. Our costs in that area are pretty minimal - mostly first aid supplies, occasional OTC remedies, etc.
Toys and gifts are soooo variable. When ds1 was little, I had very little money, and we didn't spend a lot on him. We've got more income now, so I spend more. We rarely buy toys, books, etc. during the year, but I do pick up little things sometimes (Dover pocket books, some balloons, a book, whatever). I go stupidly crazy with Christmas stockings, but I don't know the total (I need to track it this year, as a reality check!). Their actual Christmas gifts are probably about $150/child most years. I'd say the kids get invited to about 10 birthday parties a year, and we pay about $15.00/gift if it's from one of the kids, and about $25.00/gift if it's from both of them. Their own birthday parties usually come in somewhere around $125-$150. That's for a party at home - food, loot bags, ballons, cake supplies, etc. It could be a lot more, though.
Teenagers? It really, really depends on what they're into. I was a pretty low maintenance type. I had no extracurricular interests, except reading, and was happy with lots of library books, and then a friend's collection. I was pretty easy on my clothes, because I didn't do much, other than read (and smoke pot, drink, etc. - but those don't put much wear and tear on clothes). DS1 goes through clothes pretty quick (runs and jumps a lot, does a lot of Parkour type stuff - things like that). He's also had lots of interests. Some of the costs for him as a teen have been his cellphone ($50.00/month - gave it to him as a birthday present), allowance ($20.00/week), acting classes ($165/month for six months - again, a birthday gift), gymnastics (fees have been between $50-$150 each season, plus the fee for the Championship - $102.00 this year, but more when they've traveled), bus tickets/pass, week long marine biology field trip ($250, plus food), choir trip (can't remember the fee, but he had over $200 for food, since he needed travel stuff/things that would keep), Ultimate ($40/year), etc. His grad photos (total scam) cost over $300.
Incidentals. Pizza/sub/smoothie/hot dog/whatever days at school - $5.00ish a pop. School photos - depends on what package you buy, but realistically (here, at least), at least $25/year. Trips to the zoo/amusement park/tourist attraction of your choice - sky's the limit! Restaurant meals for kids, if they order off the children's menu, come in somewhere around 1/3-1/2 an adult meal, ime...but when they get to 10 or 12 or whatever, you're paying for a whole other adult.
The cost of kids varies hugely. We don't do it on the high end, but we don't do it on the cheap, either. I did when ds1 was little, and it wasn't super expensive.
And, sorry if that was really long. I just kept thinking of things...
ETA: Okay - I just read the thread. It definitely depends partly on the kids. DS2 is going to be assessed/evaluated for special needs. I'm probably going to need to start paying for occupational therapy for him, at the very least. DS1 had a learning-to-drive accident last summer (totally not his "fault" - just plain not knowing what he was doing well enough yet, misjudging something due to lack of experience, and bad luck - he only had about 45 total minutes behind the wheel). The body shop bill (for our then-6 months old van *sob*) was $1700. He didn't do anything wrong, but it was definitely a hit. We're not paying for post-secondary education, so we don't have that hit. I did miss craft supplies - lots and lots of them, although I include quite a few of those in stocking stuffers and Easter baskets and such.
Edited by Storm Bride - 4/10/11 at 6:06pm