It looks like the HST P/N is TA240-58005 according to the Kubota's online parts drawings. Using various online pricing sources the whole unit is listed between $4,116.30 and $4354.61.
There is a listing on ebay for a used item with the P/N referenced above, but listed for a
L3010. Of course this is used and unlikely your desired path, but it is listed for $2,500. The only reason I point this out is it looks like the HST unit is used in other modes and therefore might be available used from various sources. Or if you buy a new unit perhaps fix your old one and sell it.