Optional additional costs

After the next beta release, i plan to create a central point for optional additional costs in the code base. We have a lot of optional additional costs already implemented, but they are all over the place and do not interact well with each other, additional costs and/or modal spells: Kicker, Convoke, Buyback, etc.
I will certainly not have the energy to convert/move all of these keywords. What do you think are the important interactions that need fixing?
I will certainly not have the energy to convert/move all of these keywords. What do you think are the important interactions that need fixing?