It depends on the domain name itself too. Avg price for a reasonably good name might be $40-50. But if the name is lousy, than I've seen $20-30 prices. IMHO, aged domains are not really helpful for SERP results, just more steady. The new domains will bounce around in the SERPs, but old domains will be steadier, that's all.
To someone here on DP that like buying aged domains, around $30 if the name is crap. Personally, I would only look for names that are quality names. If the name sucks, I wouldn't even pay reg fee just because it's 4 years old.
A strict no no for crappy names. The name must indicate what the domain is for. For Example DomainGuider.com (4 yrs old) sold for $110 @ sedo and people could immediately identify it to be of the domain/seo niche. On the contrary if the name is adsasd1314141.com and is 10 yrs old I still doubt anybody would touch it. Such names often are associated with spam . Choose names wisely.
I wouldn't, but there's many people here on DP that'll buy any name just because it's aged. Gotta sometimes wonder what they are thinking!