Here's my two-cents $10:
Say we were to add advertisements to the game client when botting. The current way I can think of this being done is via an overlay. Theres added memory usage right there. Not too mention do you know how annoying ads are on ***Bot and **Bot? Personally I always baby-sat my bots and having to close the obnoxious ads just to see what's going on gets tedious. However, there are pluses, which I will get to later.
I really like the idea, but I can't see anyone making enough money to buy a chocolate bar. Just today there were 23 bots* updated. Okay, there's 2,317 bots(?) online as of this post according to our website. Disregarding the assumption that of those two thousand, a good percentage are divination farmers/bots, I don't think that's enough. I don't know about revenue and what not in terms of advertising on RS cheating and marketplaces, but lets put this to terms of YouTube as many are familiar with it. Lets say that for 1,000 views, you'd make $1. So in our case, 1,000 users on your bot = $1. Lets be honest with ourselves, 2,000 bots listed, there's no way there's going to be 1000 people using your bot alone.
Lets not forget about everyone else however. Say someone is pulling in 1,000 users on their bot. What if the other 1,000 users are distributed throughout the numerous bots we have available. There is 50 cents available for grabs at this point, referring back to the scenario above, there is 22 authors remaining. If the rest of the users are distributed evenly, everyone is making $0.02. However, I highly doubt that'd be the case. Lets face it, the market at that point would be a monopoly. Heck, if I was a premium developer I'd make my bot free just because I feel as if I'd earn a lot more, after all, it makes sense.
Enough negativity, I think I made my point clear, to some extent. The plus sides to this are, well for starters, Runemate generating more revenue towards development and giving the people who make this happen more incentive to work harder than they currently are. Obviously, the only other plus to this is developers, new or not, will be making money for working on their bots for free (technically not though w/ your idea).
If anything, I'd suggest to add advertisements, but to help out Runemate, and use a portion of the revenue to pay staff to continue working on the bot and adding new features, or simply increasing the bots performance, providing more in-depth tutorials to help people develop their own, crowd-source difficult bots such as a quester, Zulrah, and many more. Heck, maybe pay bot authors on other websites to port over their bots to Runemate. Or even better, help source a way to automatically translate other botting platforms API to our own, if its legal of course.