I bought two ballast boxes from Kubota while taking advantage of 2 implement incentives with two different tractor pruchases. The smaller one, bought for my B3350 is the same one recommended for the BX series. It's been a couple of years but, IIRC, is was well under $300.
FWIW, The Kubota supplied ballast boxes, while well made and well suited to the tractors for which they are recommended, are just that, ballast boxes. You will find no hitch receivers, tool brackets, cubbys, etc,.
There are competitively priced after market ballast boxes available with such conveniences built in.
Of course, it you are a reasonably competent welder/fabricator, you can build your own to suit your spoecific needs.
SDT