Quick answer: Split runs about EUR 90 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September-October - swimming weather without the July-August crowds and prices.
What things actually cost
espresso at a bar
1.80 EUR
bell tower and cathedral ticket
12 EUR
local beer (0.5l) in a bar
3.50 EUR
scoop of gelato
2 EUR
catamaran to Hvar
15-22 EUR one way
marenda lunch menu
9 EUR
Eat well for little
Grilled fish with blitva (chard and potatoes) - the standard Dalmatian dinner, about 18-25 EUR in a konoba
Pasticada with gnocchi - beef slow-braised in wine and prunes, Split's Sunday dish, around 15-18 EUR
Soparnik - thin chard-filled pie from the Poljica region sold by the slice, about 3-4 EUR
Cevapi in flatbread - grilled meat rolls from a fast-food grill, filling for 6-8 EUR
Marenda (workers' lunch) - fixed early-lunch menus in local konobas, a full plate for 8-10 EUR before 1pm
Money mistakes to avoid
Treating Split as only a transit stop to the islands - the palace, Marjan and Klis deserve two full days
Buying long-distance ferry tickets on the dock in July-August - Jadrolinija and Krilo catamarans to Hvar sell out, book a day or two ahead
Eating at restaurants with photo menus on the Riva - walk 5 minutes into Varos for better food at 30 percent less
Renting a car for day trips you can do by bus - Trogir, Klis and Omis all have cheap frequent buses and parking in Split is a nightmare
Transport without the tourist tax
The airport bus to the main terminal by the port takes 30-40 minutes and costs about 8 EUR, a taxi or Uber is about 35-40 EUR. The center is fully walkable, and city buses (for Meje or Znjan) cost about 1.60-2 EUR per ride.