Probably use separate tables with a conversion rate between the items. For example you could have a diamond based economy but also accept netherite scraps as a 10 point currency.

Now that I think about it, storing the number of each currency rather than just a money value would imply that they are different things, similar to thinking of a $20 bill as not the same value as two $10 bills. It would make more sense to make a seperate plugin with a shop command and let server owners just disable /withdraw.