There is no single right number, because the gap between the cheapest and the most expensive version of this trip is enormous. The categories are always the same, though, and knowing them lets you build your own figure.
Getting here is usually the largest line and the one that swings hardest by season. Flights over summer and public holidays can run several times the ordinary fare. If your dates are flexible, this is where the savings are.
Airport to city is about 35 km. The airport bus is cheapest, taxis and ride-hailing noticeably more, a hotel car more again. From three people up, the gap between bus and taxi stops mattering.
Accommodation spans dorm beds to island resorts. Location deserves more attention than price: near Tran Phu you can walk to the sea and to food, while out at Bai Dai near the airport every trip into town is a drive.
Eating is where Nha Trang is kindest. A bowl of bun ca or a plate of banh can at a neighbourhood stall costs very little and is usually better than the tourist-facing restaurants. Seafood is the line that breaks budgets — see below.
Admission is mostly low: Po Nagar is 30,000 VND, Long Son Pagoda is free, Hon Chong and the public beaches cost little or nothing. The exception is the island park, where one pass can equal a whole day of everything else.
What actually breaks budgets is seafood and boat tours. With seafood, always agree the price per kilo and confirm the weight before cooking. With tours, ask exactly what the price covers — island admission, lunch and snorkel gear are often billed separately.