To be honest it's completely worth it. I have 85 mining on my zerker pure and it gives me a very steady income of about 800k/hour mining and world switching runite at the Keldagrim mine. 85 mining is considered quite a feat as mining is considered a huge bore leaving few people with 85+ mining. Also, the price of runite ores will almost never go down and will always sustain at 17k a piece. Even better is the option of the kMiner bot from kminer.org that can bot runite mining and world switching making you 800k/hour overnight.
Funtip:
85-99 mining on only runite yields 1.3B worth of runite ores. ;]
Well, should I powermine iron, or bank it, or should I do coal? Also, I'm going to get smiting up at some point, that why I was going to get 85 mining, and sell the rune ores and buy nats and iron ores.