You can use VLOOKUP( ) for this. Type in your table somewhere, say L1
to M4, so it looks like this:

0 100
100 95
501 90
1000 85

If you use A1 to type in the quantity, then put this formula in B1:

=VLOOKUP(A1,L$1:M$4,2)

and this will give you the price from your table (Note your description
is not consistent - 550 should have a price of 90. Also, the quantity
1000 is ambiguous - I've assumed 1000 and upwards gives a price of 85).

Hope this helps.

Pete