If you want to do it with Delphi, you could look into how the JclExprEval unit works, part of the JEDI Code Library. I wrote it several years ago (it’s a little over-engineered); it parses functions and variables and can hand you back a method pointer which evaluates the expression quickly. Pass the variables in by reference, and you can change them directly and the re-evaluated expression will be calculated accordingly.
In any case, the basics of how it works may be helpful for you. Recursive-descent parsing of expressions is easy, and by building a tree you can evaluate multiple times without re-parsing. JclExprEval actually generates code for a simple stack machine, so that it can work a little faster than tree interpretation; stack machines largely restrict their memory operations to arrays and use switches for opcodes, while tree interpretation follows links throughout the heap and often uses virtual dispatch (or double-dispatch) for opcodes, so they usually end up slower.
Taking the same approach as JclExprEval in parsing but written in C#, and building up an Expression, like Marc suggests, is another perfectly valid approach. The JIT-compiled expression ought to be quite a bit faster than an interpreted expression program or tree, which themselves are a lot faster than parsing.