wireless for school

Hi Everyone

My daughters' elementary school was quoted a large amount (gasp!..20K) to do wireless on a group of portables. They don't have the budget and asked if I would help..the principal and I would do the setup) Here's the scenerio:

1. There are 22 (wood frame & siding) portables clustered in a group.
2. Each portable has 1 computer running WinXP Pro that needs to
connect.
3. There is an admin building (somewhat in the center of the portables) with a wired network connection to the main building and broadband internet.

Total Area: 125 yards by 300 Yards, flat terrain

Here's my question: Am I way off in thinking it could be done for less? I'd like to help out if possible, but I'm not a networking guru. Could this be done with over the counter type equipment? (like from newegg)
Could I use an access point in the admin building with an external antenna to cover the area? Any advantage to using PCI adapters on the computers as opposed to USB? Any advice or guidance would be great!

Thank you