This might help with trying to assign a dollar figure for thrift store goods. These are guides from the Salvation Army and Goodwill that can be used for determining value of goods donated for tax purposes. In our area, we see things more on the high side of each range, which is what comes of living in Maryland
Both guides list sheets at $2 at the low end of the range, so that's likely the more common price people will pay.
In the past, Zombie-F has said to use the price you paid when you bought the item at a thrift store. If it's been a while and you don't know, or you want to determine what might be more common no matter where people live, then the guides can help you make an estimate. Otherwise, Haunti can clarify when she gets back from DC.
Personally, I tend to go a little higher on item cost estimates when I develop a prop budget. That way, if I still come in under budget, I know I'm good