A billion dollars probably, one can always find love later, especially if he doesn't have one already. You don't really have to choose between the two, they can coexist. Get the money, find the girl you love. Simple.
Edit: You don't really have to buy her or get her using the power of money. Just fall in love as you normally would. Because falling in love with a girl with a billion dollar in your bank would be cooler than falling in love broke any day! :P This way you can also feel assured that you will give a great life to the girl you love.