Three things, and all three are avoidable.
Parking. The single largest optional cost here. Meters in the South Beach zone run about $4 an hour and are enforced from 9am to 3am, with round-the-clock enforcement in the Entertainment District. Residential streets one block off the commercial strips are permit-only and the tow operation is real. If you must drive, park once in a city garage where the day caps around $20; the 5th & Alton garage gives the first two hours free, and the 42nd Street garage north of the district runs about $1 an hour with an $8 cap if you are happy to ride the trolley in.
The beach concession. A lounger and umbrella set for a day costs about what a decent lunch for two costs. A towel and a supermarket umbrella cost nothing on day two.
The Ocean Drive bill. Between 5th and 15th Street, expect the listed price plus tax plus an automatic service charge usually of 18 to 20 percent, sometimes with a further percentage added. City rules require the charge to be disclosed on the menu, the bill and the receipt. Ask for a printed menu with prices before you sit, and buy water and snacks from the pharmacies and grocery shops on Washington and Alton instead of from the kerb.
The free-day arithmetic: walk in, bring a towel, eat one block west, and the entire island costs you breakfast.