Is there any way to transfer coins/treasure between different Skylander characters in game? I find that I'm playing with one over the others more, but would like to share the wealth with the others so they can purchase upgrades as well.
No, there is no way to exchange coins from one character to another. If I was you I would just start playing with the others. Hats can be shared though.
With Giants, I've finally found an answer to this problem!
I've got a Spyro with, like, 20,000 coins built up. For the longest time this seemed like a waste of good solid lewt. In Giants there's a shop, and things bought in the shop are used by all the heroes. So my Spyro is now my shopaholic character, used for all store purchases all the time.
It's not much, but it's a partial solution!
Better than nothing.