I have noticed that main line thrift store prices have gone up dramatically in the years since we've been in Poland (since 1994). I notice it quite a lot since we come back to the US only periodically and tend to notice the jumps in prices.
Check for "stuff a bag" sales or periodic sales.
But you are right--your price was too high for sheets! Someone was hallucinating when they priced those or they expect people to wait for the 50% off sale and even then, it is pretty high for a used sheet!