We follow a free online Charlotte Mason-based curriculum: http://AmblesideOnline.org
I didn't choose it because it's free, but because I really love it and find it to be rigorous and thorough. You can check the suggested books out of the library or find many of the resources (books, poems, music, art, etc) for free online. If you were radically frugal, I think you could do AO for $20 or less a year.
There's also a very highly regarded math program that's free online, called The Mathematics Enhancement Programme: http://www.cimt.plymouth.ac.uk/projects/mep/default.htm
We buy consumable supplies when they're on sale as loss leaders before PS starts (i.e. crayons for $0.20/pack), and everything else is purchased used, on ebay, or when it goes on sale.
I think we save a ton of money by HSing. I've checked out the lists of required supplies for PS when they're posted at the stores in July and the required items are not cheap. Things I don't have to buy:
Tissues and hand sanitizer for the class (on the "required" lists for PS)
Markers, crayons, scissors, glue, etc for the class
Teacher's gifts (unless I buy for myself
Teacher's Aide gifts
Extensive wardrobe of clothes (we're casual here at home during the week and no one gets teased for wearing the same thing twice in a week, or going to school in pajamas!!)
Required snacks for the class (usually have to be individually packaged, commercially prepared snacks due to allergy concerns)
Gasoline to drive to/from school (I wouldn't put a 5 yr old on a bus with older children)
Also, we save money on clothes because the children change into play clothes for messy or dirty activities - at PS there's no changing for recess (if they even have
Oh, and you can do a lot of fun activities for free or at greatly reduced prices. We went to Sea World for HSer's day and it cost $8/pp versus the usual $50. The rollercoasters weren't open but it was no big deal.
We also go on vacation during the off-season -- a condo rental in September/October is 1/3
the cost of a rental in the Summer or during a public school break.
Not to mention that we don't have a TV, so my children don't know about all the newest trendy toys, and they likely won't be hearing about them too often (likewise for trendy clothes). Materialism is kept to a minimum much more easily by HSing, which reduces your overall costs.