Economics of Relationship ;)
But this is not the situation with all. Some fortunate evolved species on this planet have girl friends. So, you are thinking that they are the happiest organisms on earth. Unfortunately, no. Lets see this with an example.
Poor and Rich, both are the extremes of our society. Rich and poor have their problems out of phase by 180 degrees.(Sorry, kabhi to lagna chahiye ki main engineering kar raha hun, well out of phase 180 degrees means drastically opposite.) Like rich complain of sleeplessness, and poor gets sound sleep after backbreaking day. Rich go to so called philanthropic organisation seminar to find truth of life and if poor give his time for the same, he will die before the harsh truth of life.
Similarly people with girl friend have their own problem. Their bank balance have one place less than the non-girl friend people. Bhaiyas manning the ATMs with their Chinese handsets playing 90s songs are the close acquaintance of girl friend wale log. I know a girl who never has her dinner in mess. Shahi Darbar(our college restaurant with one gravy for every cuisines. And in hurry, they can serve you paneer butter masala as chicken 65. Actually, shahi people name the dish with the whatever first name comes in their mind.)
Some data(based on the survey on my friends):
1. Long Distance: Rs.500-600
Expanses goes on mobile recharges(of both party).
2. On Campus Placement:
There are two extremes. One who go out on dinner everyday and other one who is content with Nescafe only. Their expenditure amounts to Rs. 900-5000.
(*There are some people who are after the sales girls in food eateries. For ex: Slim Shady)