Not knowing the recipe, or how the lobster is used in it, leaves us guessing at how appropriate any substitution would be. However, if the recipe calls for a couple of chopped/diced 1-lb boiled lobster tails, and you want something with some of the sweetness and a silimar texture of lobster at an economy price - look for a bag of frozen crawfish/crayfish tails.
Monk fish, pollock, shrimp, lump and or picked crab (or a a combination) would be good - just not as close as the mudbugs IMHO. Frozen would work just fine - but I tend to shy away from canned crab and shrimp because they have little flavor and are too mushy for most applications IMHO.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain