A simple solution of soap : real soap, like Dr. Bronner's unscented liquid soap and water is very effective at controlling spider mites and would, I believe, be much less expensive. It's the cure I routinely use when I find spider mites on one of my plants, and it works every time.
you might try nematodes to reduce root pests, larval stages and prevent spider mite in the first place, along with many others, thrips and so on, without the risk of taste contamination, plus using a nematode pack is a great way to increase access to naturally cycled nutrients through their consumption of other risky biology, in to waste ionic NPK etc that our plants can then use as resource. :)
you might also try other bugs to eat bugs
http://www.buglogical.com/spider-mites/
and general resources to try to time ipm strategies with emerging threats
http://mgsantaclara.ucanr.edu/