This will expand up to about 28 items, just add to each array in the correct position, and remember the first array is ordered.
=LOOKUP(F20, {0,50000,100000,200000,300000,500000,750000,100000 0,1250000}, {"no way","*","**","***","****","*****","Silver","Gold" ,"Platinum"})
Or, you could put that in a table in your sheet and just refer to it with cell ranges:
=LOOKUP(F20, $M$1:$M$9, $N$1:$N$9)Code:M N
1 0 no way
2 50000 *
3 100000 **
4 200000 ***
5 300000 ****
6 500000 *****
7 750000 Silver
8 1000000 Gold
9 1250000 Platinum