ORIGINAL: JMcDonald
It seems like even most factory .21 engines a actually pretty decent now. You might consider picking up a new one off ebay. A quick search showed that they are consistantly available under $100. But then again, you might as well also start looking at engines like the OS and such.
The cheapest OS 21 I would consider is the OS V Spec and at $300 I would not call that a budget engine.