OK, I've stumbled upon what I believe is a great deal even by Serbian (let alone Western) standards. When booking directly on the hotel's website, there's a Luxury Escape rate for a flat 200 EUR (VAT incl.) per night, including:
- Accommodation in Executive Suite
- Breakfast
- 55-minute massage for each guest
- A-la-carte dinner for two (three-course menu, fruit platter and a bottle of wine)
Note that the rate doesn't seem to be available on SPG.com, only via the hotel's own website.