Your intuition about Rentmen is 100% correct. The guys who advertise there often expect to get US prices because rentmen is primarily used by visitors from the US. You'll find that guys on mileroticos.com typically charge 200,000 pesos (about $50). Another site, skokka.com.co are even less, often 100,000 pesos, although I think it's more along the lines of the old craigslist. You also may find you're amazingly popular on Grindr where older white guys seem to be considered exotic and desirable.
Unlike Mexico, it's pretty hard to find guys who speak English. If you don't speak Spanish, Google Translate may be your new best friend. When contacting guys (they all use WhatsApp, btw), it's probably best to start in Spanish, at least until you get a rate. If you start in English, I think you'll get the gringo rates that they use on rentmen.