the forest service publishes a list of stumpage price reports. usually when you report what was harvested at the end of the year they send a questionaire along and ask what the stumpage amount was. its divided by county. gives a high, low, and average. varies widely but lets you know what the ballpark is.
here is a link, scroll down to stumpage price reports.
Buying standing timber.
So then you have to consider logging cost, delivery cost, and what the mill will pay, and the stumpage buyer gets the difference. Could make out big, or lose big.
stumpage is what you pay the landowner for natural resource he owns. the term is usually used to describe what you pay a landowner for standing wood. but if you start a gravel pit on somone elses land you would pay them gravel stumpage. the term got borrowed.