Keep in mind that these seat belts would be retrofitted to existing models, models which may not be designed to hold seat belts since, as discussed in the article, almost no states require seat belts on school buses over a certain weight. Whatever the case, it wouldn't be surprising if installing a seat belt were 150-250 bucks as there are probably a metric fuckton of regulations on what such a seat belt needs to be.

This is pure speculation of course and, as you've said, only distracts from the real point. Whatever the cost, adding seat belts would probably only make things worse.