I ran a 5 gal AG rig starting Jan 11, 2009, and made 23 batches that calendar year. I might have gone smaller if the equipment had been available because I was worried that I would screw something up. That didn't happen, and it didn't take me long to realize that I could have made 50 gallons in the same time it took me to make 5 gallons. So on Jan 10, 2010 I mashed in on my new 10 gallon rig, and have been pretty happy with it.
My 5 gallon AG rig was a 7.5 gallon stainless HLT (about $120.00), a 7.5 gallon Polarware BK (another $95.00 or so), and a 7.5 gallon SS Blichmann mash tun that cost me well over $450.00! What I am coming to, by degrees, is that you can get old 15.5 gallon beer kegs on Craigslist for $35-$45 dollars! When I completed fabricating my keggle MT, BK and HLT I only had about $150.00 in that system! And here we, are almost 10 years later.
Of course there were other expenses along the way. Temp controllers, another pump, a fermentation chamber, stuff like that. But the keggle system is by far the least expensive and most productive system you can get in the 10 gallon range.
Charlie (wishes they still made 1 bbl beer kegs!)