Mostly the sites are sold at less than x8 monthly revenue, and when still relatively new (eg 2-3 months in operation). Why? Is it because proxy site normally cannot stay long in business (banned by hosting) or domain got filtered or blocked after several months? Does it require a lot of technical or admin effort to maintain the site?
Proxy Sites have less life and generally , ends up getting blocked or something but these can be used to generate some revenue . Regarding the selling part , these are easy to make and so if you know how to make one for you then why not make one for other and sell it for some fast bucks < my 2 cents , not every sure , I am not much into proxies >
But it is not setting up a site that takes more time. It is promoting the site to have a decent visitor count.