Have you searched for the cards on Ebay? If you haven't, I'd go ahead and start there, but click on the "sold" or "completed listings" link to get a ballpark figure.
My advice would be to start a few bucks higher than that to gauge his interest.
Baseball: Trout, Altuve, Orioles(Schoop, Walker, Mussina, Gausman) What I collect: base, inserts, refractors, low cost autos. No relics. 90's-Current. Trying my hand at very low-end O's prospecting.
FB: 90's/Early 2000's Vikings and Ravens