Don't even think of doing it in SQL server!
It's possible - Evaluate a string Expression – SQLServerCentral
] - but to use the "expressions" as you have them would take a fair amount of work, and be very messy, given SQL's poor string handling.
I'd read it into your presentation language, and work it out there: it shouldn't be too complex to do if they are all just
once you work out what relation the values have to your table data!
Me? I'd not use a string at all, but change the table definition.
If your expressions are always X% + Y% or X% - Y% then I'd have three columns:
Where sign is either 1.0 or -1.0, and the percentages are already divided by 100:
.20, .21; .03, .39; 4.01, .44
and so on.
Then the calculation becomes trivial in either SQL or a presentation language:
(X * XPercent) + (Sign * Y * YPercent)
And is a lot easier to manage at a later date when a Y percentage needs to change.