formula to decrease a margin, in connection with increasing basic value

Hello!

I'm trying to build a formula to form a price-list. I have some basic prices from a supplier and want to build my prices with a simple rule: the higher the basic price is (column A), the lower my profit margin (in %) should be (column B).
Example:
Basic value is $50, my price is $75 (50% margin)
Basic value is $100, my price is $130 (30% margin)
Basic value is $150, my price is $172,5 (15% margin)
And so on...

I forgot most of what I've learned on Excel at my university (long time ago...), so I tried to do it by using simple thresholds, with "if" function:

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Unfortunately, this method has some serious cons, e.g.:
1. It doesn't look good.
2. If the basic price of 1 product is $49,9 and for the second one - $51,1 - in result you got absolutely different end values for them. I tried to put more "if" operators, but Excel didn't let me do it.

I assume that the problem is quite simple and I should find it on the web, but I didn't, probably because my poor technical English.

Please, help.

W.

Maybe you need 3 cells.
Cell A is your cost
Cell B is your price
Cell C is formatted for % and contains the formula:

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You can copy this down as many rows as needed.

Cell C will tell you what your margin is.