If you buy the clamps, unless you can find a box of them used somewhere, you could end up spending a lot. They are about 20-30 bucks a piece.
If you have welding capability, you could go really far with this.
You can often find used racks on craigslist or maybe even ebay for a ton less than new. I've been thinking about making my own too.
If you used the right wood and finished it nice that could be fun. Get some spruce or decent grade plywood and figure out some sort of butress quick-loc thingy... Plus wood absorbs a lot of vibration.... Hmmmm... ok, I'll be in the garage for a while, lol.