Seeing as the market is crazy at the moment with players going for insane prices...What player(s) would you say you got at a bargain compared to the price they'd be worth if listed on the market today? Whatever reasons you have, lets see them. I'll start:

https://www.f4football.com/players/show/224294

Bought as a 20 yr old for $37,460,372.. Top midfielder with 20G & 75A in the league this season

I've gotten a few bargains that I was able to sell for big profits afterwards:

Bought for 33.6M, sold for 187.4M 6 seasons later
https://www.f4football.com/players/show/334586

Bought for 70M, sold for 157.5M 8 seasons later
https://www.f4football.com/players/show/206918

And my best: bought for 51.5M, sold for 427.5M 4 seasons later
https://www.f4football.com/players/show/235668

According to my observations, good AM is one of the cheapest players on the market. Every season I see at least one offer on the market in the region of 40-60 million.
I bought two for about 40, and both became good football players with a 70 rating.
It is more difficult to find promising players for other positions. The most profitable purchases so far:

GK https://www.f4football.com/players/show/416063
4,5 pot/4,5 learn/5 aggression/5 leadership and only 40kk

WM https://www.f4football.com/players/show/418568
4 pot/5 learn/5 aggression/5 leadership/5 flex and only 23kk

Both were 18 at the time of purchase. They are still learning, but they should be good in the future

https://www.f4football.com/players/show/157222
AM who has been rated as high as 78. Season 31 best midfielder in France 1.1, 4-time league leader in assists. Bought for 39k. Retiring after this season, but got 5 1/2 good seasons out of him.

Mine has to be CF Alex Braun. Bought off the teamless market when he was 18 for 60,500,000. He now has 842 career goals in all competitions. 584 goals in the last five seasons.
https://www.f4football.com/players/show/196123

Bought my GK in market for 25M when 17 years old, just finished his 10th season as a first choice GK. Been +80 rating for 7 seasons now, one of the best players I've ever had while also the cheapest.

https://www.f4football.com/players/show/196890

Of the players currently in my squad, would have to be Jaden Servania (AM, https://www.f4football.com/players/show/189099). Got him from the free agent list in Season 17 at 41.6M. At his peak he was 84.15 rated. Currently 80.10 rated with 86 EXP. He has 179 career goals and 250 career assists. In Season 23 he was voted the Best Midfielder in the England National Trophy. He was the central cog in my title winning teams.