How can I automatically adjust the prices of items on a webpage?

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Hello, I hope I can explain what I’d like to do.

There is a webpage I use a lot to know the prices of the things I sell. The problem is that page displays the prices that I paid the things for but I’d like the page to show me the prices I have to sell those things. So, for example if something in that list cost $100, I’d like to have it displayed as $156 (price*1.21+35). Is this possible?? I was thinking this could be accomplished with Bookmarklets and that way I could use it in any browser but I don’t know how to create one…. maybe there is an add-on for Firefox and Google Chrome?

Thanks!

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