Most I've spent is 11.43 on a godaddy domain. I had a 99 cent coupon but forgot to use it...
I can't imagine ever spending more than $10 on a domain name.
18 Euros for a ccTLD with a discount from eurodns. I am stil using their services BTW!
For me it is around $50, i really don't like buying domain's from marketplace's i just tend to think of good names / research for a great untaken domain.