Hi lgiuffre
I could write a very long reply about using the API but the question seems to suggest an understanding in how it all works .... having said that, once you jump through all the hoops to become a developer and hook into the API and link it to your app - directly there is no cost and it also depends on your status as a member.
It is complex and I suggest you read up on the process --- via https://developer.myob.com/
There actually is no such thing as 'linking your app to the api' as though you are linking a caravan and hence up for a cost.
Sorry if I have mis-interpreted your question.
The Doc