Last week I stayed at the Samokov Hotel for 7 nights- sole occupancy of a double/twin room HB for £231- that's £33 per night.
I just had a look using a random date in March, and the same price still comes up. The hotel is clean, warm, the food is actually pretty good (and hot!), I was very happy staying there,
Otherwise look at the Borovets hotels page on this website, they seem to offer some good prices.
Greenfrog,,,,, who did you book that with please, as all the holiday companies I try seem to have very little solo holidays left, I know it's a bit late,,,just thought I'd try and see what I can get,,,