First off, I LOVE the 28ga!! I have a wonderful Rem 870 LWT. Its is VERY efficient and patterns better than most all other gages. As a matter of fact, more first time, perfect skeet scores, have been shot with it than any other gage. It truly is a wonderful gage.
BUT, You mentioned cost, it does cost a bit more and isn't as easy to find. (Not everybody stocks it and those that do likely will have limited choice of recipe) There is also NO steel shot, buck shot OR slugs available for the 28ga.
For all this as well as the cost of the barrel/gun required, I would check out some light 20's. Sounds like you already have a 20, no? Not sure, but there are a number of mfg's out there offering reduced recoil ammo now. maybe one offers some 20ga fodder?
Just a couple points to ponder.