Ok- done a little saerching- the cheapest replacement I've found seems to be on ebay-
go there and search "Motorized bicycle"- you'll get about 800 results of mostly kits and parts-
put the items per page on 100 per page, and on page three (today, results will vary) there are two listings for a pair of grips with the nylon inner sleeve for the twist throttle
(no outer kill switch assembly, and no cable- although there are listings for those too)
anyway, boygofast has a pair for about $6 or best offer and $5.50 shipping: abou $11
another vendor has some for a little less, less than ten, shipped
I think that's about the best we can do, unless you need the kill switch too, then those are more $$$.