I’m not sure where you envision your data versus your controls, but let me run something by you that’s along the same lines as your example, and you tell me if it fits or needs adjusting:
{ColumnA}
Single select
values = ‘Full’,‘Split’
{ColumnB}
Numeric
value = $ cost of expense
{ColumnC}
Formula
formula = IF(ColumnA='Split',ColumnB/2,ColumnB)
Is that kind of what you had in mind?