depending on the list, you could use a table like structure. So the quantity in one cell and across from it the number, if set up in ascending order and a vlookup.
so for example say the value you are looking up to get the price on is in A2 and the table with 0 in K2 and 10 in L2 and 10 in K3 and 9 in L3 etc. Then a vlookup like
=A2*VLOOKUP(A2,$K$2:$L$4,2,TRUE) should handle your pricing.