We turned up to Hotel Jezero on the day and were lucky that they had a free room, albeit for only one night. The double room we stayed in cost 56 euros per night, including breakfast but not dinner. It was very simple, but with a balcony with an amazing view of the mountains. The black lake was an easy 20 minute walk away.
The next night in Zabljak we stayed in an apartment of higher standard than the hotel room about 1km out of town. This was owned by a local family along with a campsite. This apartment was only 20 euros a night, with private bathroom, kitchenette etc. so I would recommend this over a hotel, unless you are desperate to have breakfast provided! This was also 20 minutes walk from the lake, although coming from a different direction (not too sure on this because we didn't do the walk from here!).