OK, well you've given us part of an answer, but back to my questions earlier "where do you think your photography is limited?". Any piece of equipment is the solution to a particular problem, so unless we understand the problem, it's going to be difficult to provide an answer.
If you're not sure what the problem is photographically, then I'd hazard a guess that you just have money burning a hole in your pocket.
At a more pragmatic level, I'd look at your options thus:
1. Your existing lenses already cover the 50mm range, so with just $100 to play with, I'd say that lens is a luxury, unless you really want to take low light photographs using natural lighting, in which case why consider a flash?
2. At the $100 price point, any flash is going to have a limited set of features unless you wish to go into the used market. On the other hand, it will be more flexible than the pop-up and can be used off-camera, but what subjects are you photographing that require this? It would be of little value for low light landscape work for example.
3. Would the money be better spent on a tripod (you don't mention one)? If you're only photographing relatively static subject, it gets you rond the low light problem, as you can use longer shutter speeds.
4. Lastly, would the money be better spent perhaps on a class to teach you how to use what you have more effectively?
As you can, see there are plenty of options here, but it does depend on understanding what you want to get out of the purchase, rather than spending because you can.
