Mayank,
if you are planning in days (which it sounds like you are), when you populate the cost plan then it will use the "standard" calendar day length. i.e. When using days It does not use the individual resources day length.
As Jerin suggests you'd need to plan in hours.
You need to switch to hours in the task views if your work effort is "Days"
click configure
Go to options in the list menu
Set to hours (from system default)
Work effort is now recorded in hours