91% is perfectly fine and you don't need to report as many posts as I did as I'm probably an extreme case. Most of the other mods only seem to have had a few hundred reports before they got asked. I also think your calculations are way off as well. Reporting a post takes probably less than ten seconds. If you spend a decent amount of time at this site (which you seem to do) then it's pretty easy to just report posts as you come across them via your normal active browsing and you don't exactly have to 'hunt' them out.
I guess that 60 seconds might be a little high, but I think that 10 seconds is probably low as well. My calculation does leave out the "cost" of shit that the staff/mods take, especially the very efficient ones (just looks at how many threads that mprep has about him). Also buying a donator account would guarantee that you have access to donator forums, while reporting lots of posts would only increase the chances of becoming staff.
I would say that the "cost" of reporting enough posts to become staff would eventually decline to zero if you stay staff long enough as you would get a portion of 25% of ad revenue plus the ability to participate in PD
