Hi and welcome @Shawn_Ryken!
I would suggest making {Monthly Fee} a formula field and use a SWITCH()-formula in it.
SWITCH({Fees},
'Exempt','$0',
'Forewarn','$5',
'Admin','$10'
)
If you’re going to use the output in another field I would recommend to leave out the dollar sign in your SWITCH()-formula.
You can find a realy handy formula field reference guide here for future use.
EDITED: Sorry, I was too fast to post / too fast reading, so I missed that it’s a multiple select field.
I’ll update with an answer in a bit.
Okay, this is maybe not all that elegant, but it does what you’re asking for.
IF(FIND('Exempt',{Fees}),'0')+IF(FIND('Forewarn',{Fees}),'5')+IF(FIND('Admin',{Fees}),'10')