SQL Server 2008 :: Text String Parsing To Apply Operators To Datasets?

I have a problem at the moment, where the client wants to be able to type in a custom algebraic formula with add/minus operators, and then to have this interpreted, so that the related datasets are then added and returned as a single dataset.

An example would be having a formula stored of [a] + [b] - [c]

and if I were to write the SQL to apply that formula, I might write something like (let's assume 1:1 relationships with the ID's)

select a.a + b.b - c.c as [result]
from z
inner join tblA a on z.id = a.id
inner join tblB b on z.id = b.id
inner join tblC c on z.id = c.id

The formula can change though, maybe things like:

[a] + [b] + [c] + [d]
[a] + [b]

The developer before me wrote something SQL-based where they parsed the string and assigned each value of the formula as either positive or negative (e.g A is positive, B is positive, C is negative, now sum the datasets to get the result), and then created one large table of values then summed them. This does (kind of) work, I'm just contemplating potential alternatives, as it is quite a slow process, and feels like it is quite convoluted, when I get into the details. If I were to do something like this in SQL, I'd normally want each part of the expression to be a column, and then to just apply the operators, but because the formula can change, then the SQL would need to be somehow dynamic for this approach.