Many engines cost up the wazoo, but one that I think would be worth it is Avalanche 2.0 Just Cause 2 and theHunter show it can be used to make huge, but beautiful, maps as well as diverse gameplay. If a small time dev was able to get their hands on it for theHunter and work with Avalanche Studios for a free-to-play game, maybe it isn't too expensive...relatively speaking, of course. CryEngine 3 supposedly costs around $1 million. Unreal is cheap, but would take a lot of work to make large areas that run smoothly. I think it would be better as a standalone, if they approached it with an open-attitude. Like, have the same mindset as if it were a mod, but make it not a mod. I mean, it sounds easy enough, right?