Are you playing on 1.4.2? I've noticed since the new beta, this is a common issue. I actually think the economy is working as intended. I think the reason for the drastic change is introduced because of the fix to wars/truce periods. The prices are somehow based on the number of goods going in/out of the town. But the big difference I think is sieges/wars would cause a lot of resources in a town to be consumed. Now that there are far fewer wars, towns seem to have an abundance of goods at all times. Making it super easy to buy 'low' but difficult to find towns that are willing to purchase at a higher price.