I'm not sure if we can link to other websites on this forum, but I know another (very large) gaming forum with an RS2 community that has a section for these services. And you don't people pay with real money, either. If you want to know more send me a PM and I'll link you to the site.
Maybe I'm missing something here, but why would you decrease the cost for buying in bulk... couldn't someone with 500m just sell you 50m, 10 times, to make more money?
The decline in bot numbers means that everything in game should go up in gold value, and gold should go up in price. It's taking a little while for that to happen but it should play out that way. Unless of course bots return.