A couple of years ago I went on one of those night flights (this was from Leeds/Bradford airport) that heads north towards Iceland and then circles round without landing anywhere.
The Northern lights were pretty good and the trip cost around £70 if my memory serves me right. This is the firm we went with.
I have been to Iceland too, for a 3 night stay It is a few years ago and the Northern Lights were spectacular.
Iceland is hellish expensive and exceedingly boring
Well there is this http://www.visitfinland.com/article/doze-off-under-the-northern-lights/ It is quite expensive though. I found another place a while ago which had a massive glass window where you could watch the Northern Lights from their bar. I`ll see if I can find it. Scandinavia is never going to be cheap though