Originally Posted by MRT2

Run, don't walk away from this bike salesman, as he is full of crap. I don't ride anywhere near 20 mph, and I can tell you there is a huge difference between a road and a mountain bike. I have a mountain bike, and have owned other mountain bikes in the past. They suck at riding on pavement. My 18 year old son can smoke me when he is on his road bike, but when I am on my road bike and he is on his mountain bike, I can smoke him on pavement. Mountain bikes are just not great on pavement. It isn't what they are built to do.

So, shocking but you have a salesman who is lying to you, or maybe he doesn't know much about cycling himself. Either way, don't do business with him.
Now, it may be he doesn't have any road bikes in your price range, as you really can't get into an entry level road bike at your budget. You need to go up by a few hundred, or buy something used.