I think the pricing percentage will depend on what Mom has to sell. If it's a lot of stuff that isn't worth a lot and a lot of hours are spent cleaning or digging through things, then yes, I can see the 30%. How much is he likely to get vs. how much time is it going to take him? How much of Mom's stuff would you buy if this was someone else's sale and would you pay very much for it?
If there's a lot of valuable things that will bring a decent amount of money, then 30% is too much.
I don't think anyone would go through a complete household for 10%. Would you clean out your Mom's house and run a garage sale for 10% of what her "stuff" sells for?
Bill