As for results, his 137 wRC+ mostly at A-ball is fine, and just turning 20 and graduated to High-A ball, he still projects as at least an average Major League player. The big question with him is if his hit tool - ability to hit for average - will rise from the current 25 to the potential 50. The good news is he doesn’t strike-out a ton for a young power hitter and he does take some walks, so his profile is actually pretty good.
His key rookie card is the 2018 Bowman Chrome Draft Auto. The last raw one that sold on eBay was mid October 2019 for $49.99 plus $3.50 shipping. We’ll call it a $50 to $55 card. The cheapest you’ll find on eBay the night of this writing is $60.
There are a couple of other Bowman Chrome Draft autos of Casas that are selling, however, that include the Class of 2018 auto (the refractor numbered to 250) and the Draft Night auto (refractors numbered to 99 and gold refractors numbered to 50). These are attractive targets because there are simply so few of them. The class of 2018 refractor auto you can find for about $40 plus shipping. The draft night auto you’ll be lucky to find for $85 - that comp is already graded PSA 10.
As a collector, I actually favor the serial numbered autos, but my opinion is that the refractors of the 2018 Bowman Chrome Draft auto are the best investment for ROI. That’s because it’s his official 1st Bowman card and the card with the biggest market. That being said, having already graded copies of extremely limited edition cards is a nice “go for broke” strategy. If you want to play it safe, there’s the non-auto 2018 Bowman Chrome Draft which you can find in high grade for $15-20.
Personally, I’d play it safe with Casas for the time being. Yes, he is the #1 Red Sox prospect, but he just hit .254 at A-ball. Yes, his batting eye is decent and he made noise when he did make contact. I need to see more progress before I would invest in his future, outside of maybe some raw Chrome 1st Bowman cards or colored parallels of his base Bowman Draft card.
All of these points about buying and selling sports cards are just my own informed opinions. They should NOT be taken as professional investment advice. Always do your own research, as the card market can shift drastically without much notice. Remember, this is a hobby, so you must be prepared to be stuck holding any cards you collect.
Transparency notice: As of this writing, I currently own no Triston Casas cards, but may purchase some Chrome base cards or colored parallels purely for Personal Collection purposes.