We're homeschoolers, too. I have my middle son signed up for a home-based Kindergarten class this year as part of our Waldorf Co-op. This particular class is in a member's home (well, outside that member's home!) and is taught by a Waldorf teacher. Four hours, two days a week. It's $45 for one day, $75 for both....so $300 per month. There is a $150 materials fee per family for the year and we haven't figured out lunches yet, so it may be more (this is brand new....we just got it together this year).
I feel this is a fair price for the teacher's expertise. Just compare it to tuition at a Waldorf School! At our local school, I just found online, the Kindergarten tuition is $13,410 yearly plus a $350 materials fee.
My oldest son (Grade One) is taking a 90 minute outdoor science class, again led by a retired teacher. That is $20 per child per class, and an arts class (1 hour) that will be $35 per child per class.
As a group, we are also looking into doing classes one of the local Waldorf schools. We anticipate that this will be less expensive.
I'm going to guess that a class for your 4YO will be play-based and that you will be staying throughout the class? Naturally, that would be less pricey. But, unless there are 30 kids in the class (ours only have around 6 children per class), or there are no materials included in that, or it's parent-taught while teacher-led, it sounds about right to me.