Validating a Session value

Hi all, I am trying to figure out a way to calculate a value based on another value and then set that value in a session variable. So, if $session_ValueA <= $100 set $session_fee = $10 Or If $session_ValueA >$100 and < $499 set $session_fee=$20 Or If $session_VAlueA >$499 and <$999 set $session_fee=$50 Or something like this., I have to evaluate $session_ValueA to 6 possible values. Once this is done, I also would like to be able to calculate a % of Session_ValueA As commission… I tried to use If, Else but can’t seem to get it right.. Any suggestions?? Thanks Aleks