Creating A Fake Discount Calculator?

I'm very new to excel, my boss request me to create a discount calculator. A cost of price is given. We have a fix discount rate of 5% of that cost value. Some new customer want more percent of discount to the item they purchase. To deal with that, a markup price will be quoted then we will make a discount according to what the customer prefer but the final discounted value will be always be the discounted 5% of our cost value. I've problem creating it because as example; the customer request for 15% discount, when I add the cost + 10% then minus it back to 15%, the value I get will no be the same of the fixed reduced 5% value of my cost value.

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