There's tonnes of smaller B&Bs about 15 minutes away just around Tavistock Square which are from around 20-40 quid. As they're all family -owned type affairs, I reckon you could come to some sort of arrangement with them regarding a discount, if you're going to be staying a couple of times a month. I stayed in one when I was looking for somewhere to live round there, and okay, it wasn't the most salubrious establishment, but it was clean at the people were friendly!

B.x

The Christian guest house:
http://www.a1tourism.com/uk/a43796.html

Not actually in King's Cross as it turns out, but the tube will get you there easily enough.

Hello, I stayed at an excellent wee B&B when I went to a conference recently. It was called The Arran House Hotel, in Bloomsbury, and it was approx £50, which is really good for London! The hotel was lovely, as was the buffet breakfast. Good Luck