welcome!
You might not be able to find a new road bike for $500. I think you may wind spending some more money(600-750US), if you want a "drop bar" road bike.
As as far advice, most bikes at the price range will probably have Shimano Sora components. The best thing you could do is to face the piranhas(you are shark, aren't you?) and do some test rides. Different makes of bikes will have different riding qualities. I would also try shopping more than bike shop, if it's not much of a bother.
You may want to look at Hybrids/Flat bar bikes. They would be more in your price range, and are bit more versatile. I know you said you were only to going to ride at the park on the road, but a hybrid can handle some mild off road stuff with aplomb, where a road bike would not be happy.
Let us know what you find out.....