Hi there we booked through going places to go to Borovets four star half board with ski packages and insurance included, was really cheap under £500, think you just need to shop around, Good Luck Gemma
I went to lastminute.com and found 7 nights to Bankso for £149 based on two people. I'm not sure if there are any left for Jan.
Also I booked a holiday (it was pretty much last minute) through Balkan holidays (sorry don't have a website or number handy) for aprpox the same price although the resort is in Bovorets not Bansko.
I did think there insurance was quite expensive so you might want to shop around.