Hello im using the follow if statment

=IF(C21250,C21*15%,IF(C21=500,C21*25%,IF(C21=10 00,C21*50%,"NO")))

it works fine but when it comes accross a number "1000" it doesnt times it
by 50% for e.g 1000*50% is ment to be 500 the if statment gives me 150 anyone
help thanks!