My first boil kettle was a 5 gallon stainless steel stock pot. I got it for $8 on clearance with an additional 15% of coupon at Ames Department store (now out of business). It served me well for many extract batches and as an HLT for a few partial mashes and all grain batches.
Before I started making partial mashes I purchased a 7.5 gallon 804 stainless steel stock pot from Kitchen Collection for $40. I used this as a boil kettle for 3 or 4 batches.
My current boil kettle is a 15.5 gallon universal kettle from Sabco Industries. This cost about $110 with shipping. I don't use a false bottom or anything, but am considering a bazooka screen. My last few batches I've gotten quite a bit of trub into the fermentor. This isn't necessarily a bad thing, but the more I can keep out, the easier it will be to make saving yeast a cleaner project.
My boil kettles:
