It doesn't necessarily have to cost money to get someone to offer a second pair of eyes. Could be a friend or interested party. You could add an attribution "

It doesn't necessarily have to cost money to get someone to offer a second pair of eyes. Could be a friend or interested party. You could add an attribution "Edited by <name>" so they get something out of it perhaps. It's pretty common amongst webnovels, translations, and fanfiction for an editor to help out and get attributed.