As long as they are staked down properly they are very sturdy. We use them when we shoot at Pawnee and we had it up for a whole weekend in near hurricane winds of 60 MPH, guy it down right and your fine.
Just an FYI, you cab get these on sale at Walmart in Parker for $50 for the 9x9 and at Big 5 for I think $60 for the 10x10. That guy is way over priced on his.