Hi TJ, I off course have not seen the inside of your steering box, but the same goes for David. It really surprises me that he can give you an idea about the cost without having seen it.
Normally I would assume that you and him were talking about different things, that happens to me not being native english speaking, or even worse doing business in Italy.
What I could imagine is that David, as some sort of warning, after receiving your steering box, after costly shipping, then finds out he can't repair it, and he is ending up replacing all the parts, bearings, seals, worm , roller, shafts, etc. And that ending up to the number you mentioned, still too much I would say, but maybe it is realistic? See it as a worst case scenario. Mind you, steering is important enough, so you want to get it right, and with David doing the entire box, you will have the best possible steering possible.
Still convinced David is the best man to do this kind of job. So in a way I am hoping this is what he tried to say?
But now back to your steering box, would I go with David's advice? No, not based on just a phonecall!
You apparently trust the guys at Straightline Steering, those are the guys that have seen the inside of your box! So I would definitely stick to their opinion, and not David's in this case. The guys at Staightline are the professionals, no reason to doubt them.
So if they say a bearing and cup is all you need, order that from David.
By the way, his remark on filling the box is important. Never use grease, only grade 140 oil.
Mike