When I first started Runecrafting last year, i thought solo'ing natures would be the way to go, but i did some calculations and learned that I'd earn my money quicker by doing fast free laws and then double natures after 91 then if I would of just solo'd. I was saving for a Green phat at the time so I wanted the quickest route possible. I made the money back that I lost at fast free laws in less than a month once I got lvl 91 and then another month later I got my green phat. Had I did solo natures I still would not have been anywhere close to completing my goal and I doubt I would of had 91 rc even. If you have the cash to do it, I'd say go for Fast Free Laws:) or maybe even zmi. I'm not sure on the time frame with zmi though as I've never used it. Hope this helps:)