Im not sure if this has been asked before, but how do you determine how many coins people have to pay for a certain amount of experience points that are put on a pokemon.

training is the way quite a few players make their coins. they see it as a business as sort.
so just like anything in business, you have to try and under cut your opposition... have a look on the market forums and see what other people are offering. i think the lowest of seen (that i know is legit) is 50 coins per bil xp

most people dont have high lvl pokes so they dont train as fast i gain 135 mil per battle so i gain 1 bil quite fast the rates for me is 100 coin for 1 bilbecasueof how hard it is to train am i right

People go by different rates. Different people charge different amounts per billion or a certain exp.