A mod requires the gamer to own the original game that is mod'ed and uses content from that, and no game company will let you sell retail/commercial mods off of their engine, neither will they for TC's.
If a "mod team" has a AAA quality mod or TC, they are best to see if they can locate a publisher who will help with the finances in licensing the game engine.
The only inexpensive routes would be licensing engines like TGE, but IMHO that isn't in the same class as the Unreal Engine etc. | 
