Building a notebook in general is a lot more expensive, and harder than say a desktop. There seem to be very few resouces for the person that wants to do it himself/herself when it comes to notebooks. I looked at the link to asus and if you are going the route of having a notebook custom made by a vendor, than I would look at a couple of different places.
Some good places to get a notebook are, asus like you have there, DELL, and Cyberpower inc. I would stay away from HP, Compaq, and Gateway at all costs! Also DELL will price match on competitors, so if you find something you like give DELL a call and see what they can offer. However when I was working for DELL they did not price match Cyberpower inc.
Price wise, when it comes to notebooks $1200 is going to put you not quite middle of the road. Extremely low end note books go for around $900 bucks, and high end notebooks can easly go for $2500-$3000+. So when making a decision, make sure you know what you want to use the notebook for now, and in the future, and plan according to that. Since notebooks aren't as expandable as desktops try and buy something a little more than what you need, that way you have something that will last, and you won't out grow to fast.
Well good luck with your decision, and when you make one.. enjoy!