I've (or at least my company have) paid ~$15.000 for a domain name. Sold six months later for ~$19.500, so it was a good investment. Investing in expensive domain names might be a risky business, especially if you don't have access to reliable traffic data. And the Right Way to measure traffic is obviously different depending on who you ask...
£9 - about $15USD Never seen the need to spend much more than the basic rate. BUT I can see the potential value some names may have for companies with deep pockets
I usually just buy my own domain names from NameCheap for $8. On one occassion I bought a domain name for $20 from someone and sold it for $60 later.