Microsoft excel price list calculations

I am setting up a print ecommerce website. I have 68 price list spreadsheets saved as CSV file from one of my suppliers. Some have two pricing columns in them and some have three.

I need to create a formula that creates and assigns a markup margin-based on what the cost is. The basic structure is:
£10000+ 115% (x1.15 giving a 15% profit margin)
£4,000 120%
£2,500 125%
£2,000 130%
£1,000 145%
£750 140%
£500 140%
£250 175%
£200 200%
£100 225%
£50 250%
£25 275%
£15 300%

However, I dont want there to be jumps from 15% to 20% margin. I need a formula that works out to the penny the markup between two monetary values. Sorry if I am not making this easy to understand. I am struggling to put this into words to create a clear explanation. To try explain by example;

if £15 has a x3 (price x 300%) markup ratio
and £25 has a x2.75 (price x 275%) markup ratio,
then for £20 the markup would be x2.875 (287.5%),
and for £21.25 the markup would be x2.7125 (271.25%)

I need to create two output files;
1 - an excel file with three extra columns per cost column. One for the sale price, one for the margin (eg 2.5) and one for the profit margin (purchase price minus sales price)
2 - a duplicate CSV file that only has one additional column with the sales price formula in. eg "Sum=A6*1.25" *This file must have the same file name as the original.
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I would like costs for two parts.

A - to create the formula / action described above and a screen video of how to perform this.
B - to run the formula on the 68 spreadsheets and create two output files for each.
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Ideally, I would like to be able to adjust the fules to amend the margins and prices easily in future.
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I tried to upload an example CSV file but this site wont allow me to upload CSV or zip file. I have uploaded one of the files as an excel spreadsheet. You will notice that three of the columns are highlighted in yellow. These are the price columns I have added to the original CSV file.

I hope I have managed to explain this clearly. If you have any questions, please let me know?

Thanks, Fran