Depends on where you're advertising, how big the audience is and how long your advertisement runs. Some sites I'd say calculate how much money you'd spend by how much people visit the site. However, that wouldn't work considering these sites have bloated visitor counts due to people clicking a link in chat, voting for a server and exiting out never even visiting another page on the site. So it would have to be calculated by unique visitors. It's also about the interaction rate of ads for that specific site and the average view time for the page your ad will run on.

I wouldn't try paying what I have started doing is just beig very very very active on minecraft sites that have forums like Spigot, PMC, minecraft forums, and a few other and my player base is slowly building. So unless you really wanting to get a boost fast i would say try this method.