I would try Turtle Brittle. Also try soaking the floating pellets in something tasty (tuna fish juice, juice from microwaved shrimp) just before putting them in the water to get them started on pellets. My two adopted (orginally WC) eastern muds did not like pellets at first, but gradually learned to eat them. I had to use forceps to move the pellets in the water to make them more enticing, much like a pre-killed pinkie for a snake. I have also stuffed pellets into pieces of cooked shrimp or cooked fish, and offered a few pellets in the water in the same area as the "stuffed" food, hoping that the feeding resonse would kick in and they'd start eating anything nearby.
Have you ever ordered from www.wormman.com? He offers a worm breeding system for European earthworms, and a friend of mine has had good luck with it, in fact she usually has more worms than she needs for her box turtles, just from her one bin.
Insects are relished by my muds - everything from crickets and superworms to hissing cockroaches. If you can get a starter colony of roaches, you could probably save a lot of money.
My muds also like greens, on a mild to moderate level. They seem most fond of water hyacinth, but won't touch water lettuce. They don't eat greens like a slider or cooter, but do nibble on them.
I think a high quality pellet with some supplimentation of greens and varied animal protein is probably the best way to go. Oh, and don't forget cuttle bone as a suppliment. My male won't touch it, but the female devours it.
Katrina