What are the best power ups that give you the most bang for your buck

Kenz said:

I need some players to power up that are cheap in terms of training and are decent cards

Training costs the same regardless of the player. If you're building a team on a tight budget, I'd actually advise against powering up anyone.

If you build your lineup here and post your budget, I can make some recommendations for you:
https://www.mut.gg/lineup-builder/