One way you can turn coins into stamina...

This is only if you have a huge coin stack, and are very close to making your last TOTY, but just need a little more stamina because your gems are drying up.

Buy Pro Packs, go to the Madden Today Sets --> NFL 100 Exchange --> NFL 100 Grab Bag. You can trade in 10 75+ for the grab bag, and I ended up getting stamina every second or third grab bag. You also get badges for whatever NFL 100 players are still on your roster.

I did this over the past week using 75-78 in this set, selling everything else I could in various ways, and I just about broke even for the week, but it allowed me to build 14 TOTY overall - all four ST, all four LB, all four WR/TE and two of the new 98s.