The biggest transfer I have ever made is 200mil for a regen striker. I had more money than sense. When your transfer budget is almost 500mil though 200mil is not that much.
The biggest transfer ive seen between 2 AI clubs is a 65mil deal for Gareth Bale who signed for PSG from Madrid after a couple of seasons.
Don't get me wrong, I see plenty of 40-50 offers and transfers. Had an offer of 72m for one player which I accepted and now the 107m. It doesn't happen often though, which is fine.