Prices for accommodation in Marbella are quite high - so, if you want to stay in one of the 5-star hotels, count on 250-350 EUR per night in a double room. In the Old Town area you can rent a good room in a 3-star Marbella hotel for 70-90 EUR (in the "low season" it is quite realistic to find a double room for 50-60 EUR).
The most inexpensive accommodation in Marbella - in one-star hotels, the average night will cost about 40 EUR. In a 1-2 star hotel in the Old Town a room will cost within 30-70 EUR (for example, in Pension Aduar or guest house In House Marbella). There are also budget establishments - so, the cost of accommodation in Marbella in the hostel Albergue Inturjoven Marbella (the very center of the city, 1.5 km to the beach) will cost 15-30 EUR.