Yeah, I would say for a 5 gallon batch, a 10 gallon kettle and a few drops of Fermcap is pretty darn perfect.I usually collect about 7-7.5 gallons of wort for my "five gallon batch" (which actually results in closer to six gallons in the fermenter), so I wouldn't think of going with anything under 36 quarts. I actually boil in a 40-quart (10 gallon) kettle. I also use Fermcap in the boil.
There are decent deals out there. It doesn't make sense that a pot that is half again as big woudl cost 3x as much. Here's an 11gal one for $89:i don't think the price difference is "minimal", but it's worth the investment for sure.
I bought my 7.5 gal pot for $70. Cheapest 10 gal I can find is close to 200