Hello again Meg,
The reason, as I understand it, that vinyl windows are so expensive is that they are very efficient and durable. I have a feeling that the whole supply and demand rule comes into play here.
As far as the cheapest, I don't know. What I do know is that in the world of home improvements you generally get what you pay for. This is true with laminate flooring, appliances, and most certainly doors and windows. I would check out Sears and also a distrubitor for a well known company like Jeld-Wen and see just how much more they are.
Good luck and keep the drafts out up there!