Leaking Tran.fluid,possible seal. Whats the cost to repair, if it is the seal?

We replaced three trans pan gasets before it was discovered that the leak was from the little O-ring on the dipstick tube where it goes into the pan. I don't remember the gasket replacement cost; I do remember that it can be a very mess job. The O-ring replacement was very easy. This info probably to late for Miss Jade, but it might help someone. jj