Systema plans to release a new line of Systema "Complete" gearboxes that come equipped with everything you need to simply drop into your AEG. Troy sent along a few tidbits of information about this:
Systema Mech Box List
Systema Mechbox Retail Price List
They range from $230 up to $280 on price. You'll need to convert the currency to find out costs on specific models. Troy has access to get these if anyone is interested, drop him a line.