The answer to this will depend on the country that you are operating within. Most countries should have some sort of Performing Rights organisation, the exact name may vary from country to country, and they should handle things on behalf of the various publishers and/or owners of any copyright.
Note that YouTube may be aware of your operation, and they COULD close down your channel - but this may be dependant on one or more copyright owner making formal complaint to them. This could depend on the scale of your operation.
Cannot comment on the price you might change, this will depend on the quality of the midi file that you create. However, I would suspect that the higher the quality of the product, and/or the more you might charge, then the more likely you are to upset the legitimate owners of the music such that they may take action against you.