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:

=IF(A1<50;A1*3;IF(A1<150;A1*2,85;IF(A1<300;A1*2,7;IF(A1<500;A1*2,55;IF(A1<700;A1*2,4;IF(A1<900;A1*2,25;IF(A1<1200;A1*2,1;IF(A1<1500;A1*2;A1*1,9))))))))

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.