You'll have at least four expense items: hotel (hostel), transport, meals, and entertainment (including shopping). Do the math for yourself on the cost of a Miami vacation:
The pearl of Florida tops the USA ranking of cities with the highest cost of hotel stays - Miami vacation reviews confirm this. On average, a night in a three-star hotel in Miami will cost 150 USD, in a five-star hotel - 300 USD, and for a room overlooking the Atlantic you will have to pay at least 1400 USD.
Renting a car will cost from 80 USD per day. The cost of one bus ticket is 2 USD. You can save money by taking the MetroMover in Miami for free.
Food here is varied and affordable - you can eat to your heart's content for an average of 10-20 USD. For example, for $13 USD at Tobacco Road (South Miami Ave) you'll get a half-pound portion of mashed potatoes with lobster and salad, and chicken and rice for about $4 USD.
If you're planning a budget vacation for two in Miami, you can cook for yourself - there are plenty of Russian grocery stores, such as Marky's Caviar & International Food on 79th Street or The Fresh Market (you can even find buckwheat at the latter).
The most "tricky" expenses while vacationing in Miami are shopping and entertainment. You'll have to pay 8 USD to rent rollerblades, about 70 USD to rent an ocean fishing boat, and the average cost of a cocktail at a good club ranges from 15 to 20 USD.
