I spend $15 or less. I also buy gifts in advance. Like Marshall's sometimes has pretty nice kids toys that were originally in the $20-25 range, marked down to 12.99. When I see something good, I buy several and keep them in a closet. DS has been invited to quite a few birthday parties for his preschool classmates and it's been very convenient.
I see that what I spend is kind of on the high side compared to other responses. DS goes to a kind of pricey Montessori preschool, honestly I don't think it's that expensive but it does cost about 50% more than the other local options (which are all church preschools). Anyway I have been surprised by some of the gifts given by other children at these parties. They usually look quite a bit more expensive than what we bring. So until now I felt like I was the cheap parent!

Good to have some perspective.