I went out on a Skidoo last year in Borovets. Booked it through a holiday rep in the end, but it was the same price face-value direct from the skidoo operators. Maybe you can haggle, especially if a big group. Facing up hill from the Rila, head left to where the other chair lift and the black run comes down. They were based out of a wooden building on the left. Cant remember how much it was. They lead you up the official(?) skidoo path up the side of the mountain and back.