I understand it would depend on the size, but what would one expect to pay for plain/friction bearing brasses?

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For the set?

Weigh one brass x 4 = total brass weight. Get brass price for the specs you want and then a rough cost is known. If you ask a foundry, be sure to give them the specs without Babbitt, which is a different process. Best bet is contact known used suppliers to find a set that matches, but expect to pay for a set with Babbitt and not at scrap Value. Not cheap either way. If you know a scrapper of cars, you may luck out at close to scrap prices, condition as is.

I have scrapped cars in the past and salvaged the Babbitt for reuse in renewing old brass.
Had a lab report done on the Babbitt to assure it had the right properties. Saved OERM a lot of cost by shipping them the Babbitt. If you are near a large Foundry, as I am in Dallas, I called and asked what they did with excess Babbitt from re-babbitting brass. They never
reuse and always buy new Babbitt and was willing to sell me the new scrap at a very good price, compared to new Babbitt.