How much money would it cost for Elton John to help me write a song for me? ?

It would likely not cost anything, provided you managed to contact him, and he liked your sound, and he was willing to make time in his schedule, and if he could fly to you, or make you fly to him. Basically, it'll be really hard to contact him, much less have him write a song for you, as even he has a producer who helps with the basics of the song's format. However, good luck.