I don't know of any mid-price hotels in South Beach, which is a relatively expensive, trendy destination. Anything reasonable is going to be blocks from the beach. Why don't you give the Hotel Reservations Network <http://www.hoteldiscounts.com> a try to see if they can find something for you? They offer good hotels at discount rates.
If you can nab a special rate at the new Loew's Miami Beach, go for it. This hotel is beautiful and has a great location right on the beach. Also, if you're willing to stay up the road apiece in North Beach, you might be able to get a better rate at a beachfront hotel. I stayed at the Wyndham in May and it had a large rooms and a beautiful spot on the beach.