A lot of standard ATX cases fit at least 6 hard drives, you can fit more if you use hard drive mounts in the DVD drive bay area.
You can also get something like this:
http://www.amazon.com/SABRENT-3-5-Inch- ... .5+adapter
Which allows you to fit two SSDs in 1 Hard Drive bay. So if you had 6 Hard Drive bays, you can fit 12 SSDs. It's a good space saver. SSDs don't generate as much heat as Hard Drives, so it should be okay putting two together like this. Especially since most cases have fans in the front anyway.
I'm assuming you're custom building a PC here and not buying one of those brand name garbage.
SSDs comes in different sizes than the standard hard drives:
60GB
120GB
240GB
480GB
960GB
Currently the lowest price you can get for 480GB is around $280 USD + tax.
That's already pretty good value. You can't compare this to a 500GB HD that sells for around $70 because again, the performance is night and day.
It's like trying to compare a watermelon to an apple. They've dropped significantly in price already. The SSDs use to cost well over $1 per gig.
For about $80-100 you can get a 120GB SSD now, and for about $160-200 you can get a 240GB.