Make a little table somewhere showing the start of each spending range and the number of points for each range, like this:

0 ... 0
501 ... 100
1001 ... 120
2001 ... 150

Suppose you put this in X1 to Y4. Then if your monthly spend is in column M, starting with M2, then you can have this formula in N2:

=VLOOKUP(M2,$X$1:$Y$4,2)

to return the points. Copy this down as required.

Hope this helps.

Pete