There was some mention of the cost of vaccination when Brookfield first had IBR - and before it was known the other herds had it. I think it was around £2K for all their cattle. Then there would be vet call-out fees, and the cost of dead cattle. It's going to be similar to the cost of the tractor, if not lots more.
I'm not surprised they aren't giving exact figures, though - it's been mentioned many times that Ambridge finances are usually a bit vague (all Helen's legal fees, how the Grundys are always so broke, how can Brookfield afford to employ all of them, etc, etc.)
I think they are signposting financial woes for Brookfield without being specific, though. I think David is relying on good will, for Tony and Brian to submit reasonable costs, and if David says, "That's more than expected, can we pay the rest after harvest," he'LL expect them to agree. He's not expecting to factor in Tom being an utter git, despite enough evidence in his own family that the next generation are not all kindly and collaborative to their neighbouring farmers, whether or not they are family.