I don't know why this is still up for debate. The OP already stated that the roommate said she will not give her money, and yet the OP is asking if she should ask her roommate for money? Chances are that the girl doesn't have the money even if she wanted to give her a couple hundred towards it (half the price, or whatever the OP was asking for), and it's unlikely the girl's parents would give it to her.
I still agree that the roommate owes nothing and should have just been locked out of her room to prevent this whole problem. But, since the roommate will not give money, the OP is wasting time trying to figure out how to get it from her, unless she chose to steal it and is looking for justification. Otherwise, what will she do, show her roommie CC and tally up which position wins out and then persuade the girl with peer pressure?
Now people are referring to opinions as stupidity - I'm surprised this thread wasn't closed yet. It is all a matter of personal moral obligations, which are...personal. Personally, I would not feel obligated as the roommate to pay for half of the laptop. I also would not have felt it necessary to get upset with my roommie if she chose to lock me out of the room and I would try to remember my keys from then on. These are all just lessons (hopefully) learned.