I have found cheap kettles at www.spikebrewing.com. Many of the kettles are 1mm walls and bottom. That seems thin to me. Is that thick enough for a 15 gal/60qt kettle?
Bayou Classic pots are 1 mm, Blichmann are 1.2. I scored a B/C for a song, 1mm has certainly been adequate and has not suffered any damage in several years...your money, your decision.