Took me around 16,000 for Bagwell and around 30,000 for Trout. Haven't yet gotten Harper since I usually wait until I collect most of the free cards before spending gold on them. I suspect he'll cost around 20,000 - and honestly, if you're going to spend 20,000-30,000 for Harper, you may as well buy 10-15 legend boxes for that same prize for a chance to get a better player!
As Cheeseman said, you can get them for cheaper (~6,000). I wouldn't expect you to pay any more than 35,000 though unless you're buying every card for 300 gold.
For Trout, it took me 20,000 gold (200 cards) to get the final 50 silver. I had 1,000+ of gold and bronze, but I couldn't get the final silver. I kept picking the left card repeatedly over and over figuring that it would come up sooner than later...after 15,000 or so down the drain, I changed up my strategy and got it on another placement. Can't say there's much strategy to it!
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As Cheeseman said, you can get them for cheaper (~6,000). I wouldn't expect you to pay any more than 35,000 though unless you're buying every card for 300 gold.