How hard would it be to fake an alien encounter? Would it be worth it?

The short answers are a) pretty hard     and    b)probably.
Another good question might be whether or not the public would believe any "alien footage", even if it were truly real. Herein lies the problem for anyone with an actual alien they may have filmed. After all, what good is alien footage if no one believes anything anyway?
The reality of the situation is that most people won't believe anything unless the real deal were to present itself to them in the flesh. Until that happens, the best we can do is the footage that has found its way to circulation around the web. Here is a MUFON conference recording of what is claimed to be an alien captured on surveillance camera at a private home.  If anyone is to be believed when it comes to whether or not an alien video is authentic or not, the geeks over at MUFON should be believed. This will give you goosebumps.