It depends where you go, the seaside is pretty good, if you go to Varna you can have a good time. There are some nice hotels/resorts there, you can go in the cheap (I had a room for £6 a night) or go to the big, nice hotels that are right on the beach.
I had a lot of fun but I was there with my girl so it didn't really matter to me where I was or what I was doing
You can get to Varna with the bus, which takes about 8 freaking hours, on top of that the road is all bumpy and it's impossible to sleep. Or, you can pay £60 for a plane ride (do this if you don't have much time). Assuming you are going with a travel agency they probably include this in the price.
Also will you be going with friends, family, missus...? If you are going with just mates you can have a great time, there are loads of tourists there, as well as sky tv and cheap pints
It's obviously pretty hot in the summer, the seaside is not as hot as Sofia, but don't think that it's gonna go any lower than 30 during the day. The sea is also nice but it can be pretty rough, when I was there, on the last day, the had to close off the beaches because of the huge waves.
As for the locals they where all nice and friendly, but I was behaving myself
I went to Bulgaria for about two weeks, I spent there just 350euro, most of the food and accomodation I didn't pay for, but you could probably go there for 10 days have a good time and not spend more than £200.