What you want to do is speed up your excel sheets, which probably use excel calculations and VBA macros(?). You do not really need anything low-level, so I would recommend a high-level language.
As for your offer to split profits, I'd rather recommend giving the task away to a freelancer with a fitting contract. It's cheap, fast and you are on the safe side. From your description, the code seems to be rather short, so no need to think about architecture or similar, so giving it to a student is what I'd do.
If you want to do it yourself, though, here are some languages I can recommend:
It's the language of the web and simple web servers. It is always in the focus of everyone doing web dev and people strive to make it really fast. It's easy to learn and use and enables you to easily make the algorithms available in a CLI and web context. There's a big ecosystem with packages for reading and writing many file formats, for example excel sheets.
Python is a language, which is rather old and mostly used in a scientific context. It's also easy to learn and use and has a lot of great packages to get things done. On top of it, Python can be compiled to a binary for greater performance.
If you want to stay in a .NET context, but add a little speed, C# or even F# are the way to go. They are very easy to learn and use and compile to byte-code, running on the .NET VM. Since they are .NET, they have everything you need to interact natively with Microsoft's file formats, like excel sheets.