Hi .. ..
from Delhi you can get a direct bus till dehradun then from dehradun you can get another bus to mussoorie .. for 2 people you can get a normal decent hotel in around 2500 to 3000 for 2 days your fare from Delhi to dehradun will be around 500 per person in a normal bus so around 2500 rupees will be the fare for the 2 sides for 2 person ..
rest your meal for 2 days and local transport for 2 people .. .. total for your trip atleast you should carry around 10000 rupees with yourself
rest maybe you can finish this trip between 8 to 10 thousands