What are you basing your numbers on? If you change trainer with 35,5k salary for 18k salary trainer, it would take 16 weeks to pay off.
I did a little math and created an excel app. There are a few assumptions thrown in with some algebra that I did. If you have a better method, I would be interested. If I remember correctly, some of my assumptions were:
-you are replacing the trainer with the same level and same specialist.
-you keep the trainer until his salary is equal to the guy you are replacing
(so if you want to keep him for a shorter or longer period, this method does not work)
Run of the Mill Canadian Manager