You didn't say what material you want to build your cannon barrel from, but I wouldn't think that the raw material for a cannon barrel that size would be too expensive. You may even look into starting with some heavy wall tubing. I use to have some 1 1/4" OD x .500 ID heavy wall, seamless, 4130 tube that probably would have made great cannon barrels.
I have been working on a 1/5th scale Model 1841 12 pounder Civil War cannon for a couple of years off and on. I used stainless steel round stock for my barrel just because I had it on hand and it was an appropriate size for what I wanted to build.
Ted