An outstanding diorama builder we have here locally sells many of his finished builds in the $70 to $100 range for his smaller pieces (1 or 2 small vehicles and up to 12 figures) all the way up to $200 - $300 for his larger pieces like a recent 36" x 18" railgun and locomotive at a rail depot he built. However, and I should stress this, he does NOT charge for labor! His only motivation is to recoop his expenses to build the next diorama.
I guess the simple answer is to charge whatever the buyer will pay. I know that sounds simple, but our builds are really no different than any other piece of artwork, collectible, or valued item...the market will ultimately determine a price based on the desire of someone to own the item. The ideal situation is for 2 or 3 people to ALL want the item very much and the price will rise as they each try to over bid the others.
$10 per hour doesn't sound overly high-priced, but you may find that even at that rate, many people may be priced out of the negotiations. My advice would be to give it a shot and see what happens...