Abarxax,

In order to get the appropriate discount rate in line with your latest posting I suggest you go with a revamp of Pecoflyer's lookup formula.

Assuming the Units Sold to be in B2 then ........
=LOOKUP(B2,{0,11000,26000,50001,100001},{0,0.05,0. 1,0.2,0.3})
will return the appropriate discount as a decimal fraction.

It's up to you as to how you need to use it. Eg whether you need to display it in a cell of it's own (Formatted as %) or whether you just need to use it as part of another formula, directly, in order to compute the discount value and /or the discounted selling price.

Eg if the Unit Selling Price is in C2 then the discount value is.....
=C2*LOOKUP(B2,{0,11000,26000,50001,100001},{0,0.05 ,0.1,0.2,0.3})

or the Discounted Selling Price is .......

=C2*(1-LOOKUP(B2,{0,11000,26000,50001,100001},{0,0.05,0.1 ,0.2,0.3}))