I would suggest an expression like:
If ( value < 400 ; index = 1 ;
If ( value > 999 ; index = 3 ;
index = 2 ) )
In a script you might use: If, Else If, Else, End If.
Note that the way you've phrased the question does not specify any result for values of precisely $400 or $1000.
Alternatively to the nested IF scenario, explore the CASE function.
value < 400 ; 1 ;
value > 999 ; 3 ;
Case is a natural replacement for nested " If's ". Both will work.
Keep in mind that the Case function will stop when the first "true" condition is hit.
Nested If's will continue through the entire nest.