Why don't you search Craigslist. There a plenty of people practically giving center channel speakers away. Beats have to spend money on MDF and all that goes into designing a proper size box,porting if needed, damping, etc etc.
If you have two 8 ohm woofers you may be better of learning some speaker building skills and then work on some surrounds. They are not as critical to the surround sound experience and you can build them on the cheap. just my $0.02