It's a lot easier to find accomodation in Sofia than say, Paris (and cheaper to). Really. But you need to pass through a real estate agent agency, or it will be a nightmare, like vn said. Most of these agents are, well ... bulgarian. But you're lucky, I know a nice one who speaks well english and helped many expats : wusbg@abv.bg

As helen said, don't forget to look about transportation in Sofia when choosing your appartement. For an european city, public transport is poor in Sofia, and there are traffic jams (but not more than other european cities of the same size).

The price range is more like 600-900 levas for a nice furnished one bedroom apartment in Sofia centre. Most landlords negotiate the rent price in euros (300-450 €).