Chain Reaction Cycles has a dirt cheap brand x seatpost in various sizes for 8 quid. Can't imagine you'll find much cheaper than that.http://www.chainreactioncycles.com/Sear ... rtBy=Price
SR Sakae stuff was really solid and still good value for money in most cases. It's very well made but doesn't have the sex appeal of campag so it's much more affordable.
The only problem is that some of the older models had a clamp system that I find fiddly to use (but then the same goes for Campag too).
Probably the main factors in price these days are the prestige of the brand, the size of the post and the condition.
Some people want an authentic build and are prepared to pay daft money for the right component. Rarer sizes like 25.4 and 27mm are more expensive. Posts also tend to get scratched up during installation and adjustment, so pristine examples are rarer.
Weight was the big factor in the old days, although as I mentioned, better manufacturers also gradually developed easier to use clamp systems.