wow you guys are spending A LOT of stubs on feeding. I don't know where you got those numbers. In my case, my guy was already a 99 diamond when I started grinding/feeding for Ichiro etc. FYI, it doesn't take as much as they said it would to get to 99. The difference is you get to 99 reasonably fast this year, but your stats aren't necessarly leveled out, far from it.
But back on topic, in the example you cite for Fielder, I'm at 75% right now and spent a grand total of 4k from 0 to 75, using only very cheap cards.
I recommend just feeding low level bronze (don't bother with common). It's much more economical and it levels up fast.
I suspect they have a total number of cards set for each player, depending on the card type. For example, fielder could be 200 bronze cards. Something like that.
That's all theory, they obviously don't reveal the math behind it.
EDIT: now at 96%. So close it makes me anxious, can't wait to use this beast.
But yeah, I noticed another thing: Once you get past the 50% mark, it gets easier and faster to progress, it takes much less cards to go from 75% to 95% than 1% to 10%...