Mark Ballas Won't Charge Kim Kardashian

Mark has been hitting the ballroom floor with Kim and her fiance Kris Humphries in preparation for their GARGANTUAN wedding ... where the couple plans on performing a waltz for their first dance.

We''re told K&K have been practicing up a storm over the last few weeks -- but Kris "is a perfectionist" when it comes to his footwork and has INSISTED on more training.