Funny, I was there just yesterday.
The county actually imposes the landing fee and it is very reasonable, we paid $28 for a DA-42.
The FBO will charge an additional handling fee and a parking fee on top of the landing fee. Expect to pay in excess of $100 for a light single at Signature.
AirNav: KMIA - Miami International Airport
Go to the "General Aviation Center" instead, they only charge the landing fee.
Apart from restrooms no other facilities though.
GA usually lands on 8L, expect to turn off a "Z" and the building is just to your right (GAC). http://184.108.40.206/d-tpp/0910/00257AD.PDF
It's a little easier going into Miami Int. if you file IFR and you can expect to be told to keep your speed up until short final.
Yesterday they were using 8L,8R , runway 9 and 12 all at the same time for arrivals and departures.