Your last sentence carries the most weight. You know how much a rating will cost, and you know how much you will be paid (which we do not), so the ROI math is easy. Balance that with the reliability of the company (will you REALLY be able to work for them at least that long) and your desire to take that job. If you have no other job offers, the decision should be relatively easy. If you get another offer in the mean time, you'll just have to decide whether to continue or jump to that one.
That said, I seriously question the integrity of ANY company these days who requires their pilots to pay for their own training, especially in an obsolete airplane...