^^ You can charge whatever you want. But don't be suprised if people from OUTSIDE of YOUR CITY, or people that don't work at McDonalds or Burger King and don't know any better, chuckle at you and think its a funny joke when you tell them your prices.
No experienced rapper is going to pay you your $300-500 when they can go get a Big Squeeze, FunkDaddy, Torrie Ward, Jonathan C, etc. beat for the same price. That's not me sayting that, that is just the way the game goes.
Now, if your $300-500 INCLUDES recording the song, then what we are both saying needs to be re-worded. Some producers include studio time/mixing in their prices.