Err...That's a bit of a tough question to answer, and depends largely on how this company does what they do.
First off, I should say that $500 for adwords and management and a website...just doesn't seem like enough. I would be worried about quality.
That said, let's give them the benefit of the doubt and say they're awesome! What should you expect?
Well, that depends on your market and your approach. Your strategy in AdWords, and the amount of competition in your market, will determine your cost per click. That, in many ways, will determine your results.
Let's say your average cost per click is 5$ (quite low for some markets, but a nice number to start with).
That means that over the course of a month, your $300 in ad spend will get you 60 clicks.
Now, let's estimate that 10% of those clicks contact you (that's a high number to start with, but I've seen it happen....more typical might be 5%).
10% of 60 is 6, so you'll get 6 leads over the course of a month.
Now, using the old rule of thumb that it takes 30 leads to close a deal, you should expect to close roughly 1 deal every 5 months, on average (often, lead quality is higher online, so you might find this number to be closer to 1/20 or 1/15 - hard to say).
You can run these numbers using your real data - this company should certainly allow you to log in to your own account and to see, for example, your actual cost per click, cost per conversion, etc.
Hope that helps!