Hi Tessa!
I'm SnapApp's Marketing Manager (and Marketo power user) and you can definately do what you're asking for with our ROI caculator-- no custom dev work requried. We call it an "ROI caculator" since that's the most common use case, but really you can do any equation that you want. If you want more information, just let me know and I'll connect you to my team.
Cheers!