depends on the extension to register a new domain prices are around the below figures .com, .net, .org - $8 .info - $2.50 if you are thinking about buying domains which are already registered price depends on the domain name... ranging from few $ to millions of $$$$$$$
I am assuming that you're talking about already registered domain names and you want to buy them for the purpose of reselling for profit. It entirely depends on the name and the extension. Popular words with high number of searches per month tend to be more valuable than less popular names with less number of searches per month. Similarly, niche also affects the price of the name, the more commercial and profitable the niche is the more valuable that domain becomes. Another thing which has influence over the price is the extension of the domain, .com is generally most valuable followed by .net and .org. ccTld names are valuable as well depending on the name and which country it belongs to.