T.,
The cost of preschools do vary widely. My 3 year-old son will be starting preschool this fall (2 days/week from 12:30-3) and our tuition comes to around $1,400. Granted, he will be attending a cooperative preschool which means that parents help out in the classroom once every 6 weeks or so and help out in other ways (fundraising, maintenance, etc.). Other schools may cost more or less depending on the type of program and location you're looking for.
I did a lot of research because I was looking for a school that emphasized learning through play as opposed to a more academic style. One resource that might really help you get an idea of what's out there is a preschool guide from PACES that lists several preschools in different areas. The guide can give you information regarding teacher/student ratios, class schedules and tuition costs based on age group, teaching philosophies, description of the faciities, special features or programs they have to offer, etc. The link to their website with information on the preschool guides is:
http://www.pacesmoms.com/special_events.html
Hope this helps...