Depending on the configuration, it may be worth it. I stepped on one one time and cracked the screen beyond repair. Forced me to purchase a new one. I suggest if you decided to purchase a new one to buy the accidental protection. Sounds as if your girlfriend is a slight clutez. Either handcuff her hands to her pant's belt loops or for sure buy the accidental protection.
Screens shouldn't run too much, especially if you didn't get the SWXEGA#$%EWR screen (I know thats not a screen, but they have so many of them... AHHHHHHHHHHHH!). I would guesstimate 200 bucks max for a screen, but then, the real price comes from them taking apart the laptop to fix it... might want to see if there are people online who tried it. Might save you a couple hunderd bucks.
Yes, $800 is about what I spent replacing a laptop screen at work last year on a Thinkpad. The screen is most of the cost of a notebook. If you can find a good deal on a replacement, it is pretty easy to replace it yourself it you are reasonably mechanical. The screen just goes in with probably just one ribbon cable. The hard part is working out how to open the shell and uncovering all the tiny screws.
If fact, I'm writing this on a Thinkpad T23 that has a "dead" screen (no backlight). So I just have a monitor hooked to it. I don't ever take it off the desk anyway so I don't need the built-in screen. In any case, as mentioned above, you may be better off going that route and buying a new laptop if that Asus is more than 2 years old.