I agree with the above, mix the food it likes with the food it needs and slowly adjust the portions. I have a little, cheap food processor that I grind everything in for my iguana. It lets you mix food together as well as lets you chop up stems from the leafy veggies.
If your grocery is like mine, you'll find big bags of collard greens or mustard greens in the salad section of the produce aisle. Those two are excellent choices for your iguana...and they're cheap! Chop it into fine bits, mix it with a few other veggies, and see if your iguana likes that. Chances are you'll need to freeze a portion of it so it doesn't spoil before you use it up.