Chris, I would steer clear of Radio Shack for drivers... for what you'd get, they're overpriced. Check the sales at
www.partsexpress.com or
www.i-mcm.com for some cheap drivers.
Building a center channel for $35-$40 isn't impossible. I built a set of mini bookshelves for probably around $60-$70 (would've been cheaper if I hadn't finished them in oak veneer). I found a set of 5 1/4" poly-coned drivers with rubber surrounds on "ultra clearance" at mcm for $7.99 (they've been discontinued since, however... sorry)
MCM has since changed their website and they don't seem to cater as much to the speaker builder anymore.
I too was leery about building my first speakers... I didn't want to spend a bunch of cash to have them sound terrible. But, I bit the bullet and built a set of
Georgia Tech 2-ways that cost probably a total of $200.. they sound great and really built up my confidence to tackle a couple of self-designed projects.