Build me a damage calculator

I would like you to build a damage calculator for a game I’m playing.
I wish to access it using a web browser, so it should be written in HTML/Javascript.
I prefer efficiency over look, it doesn’t have to be fancy as long as it’s capable of computing all the possible combinations of up to 7 dices (823k max) and export the outcome in .csv/.xls format.
I do not wish to release it publicly (unless you want to) and I would like to have access to the source code (rather than work remotely on one of your server), so I can update it in the future each time new contents are added.
I have a limited budget and intermediate programming skills so even if I value your time, I ask you please to ignore this post if you think this is too hard for you or too expensive to produce.
I’ll be happy to send all the details privately together with a sketch of what it should look like and what the outcome should should look like.