Laptop prices have come down dramatically in the last few years. I just purchased a top-notch travel notebook for $1600. That's a far cry from the middle of the road laptop-brick that I scraped to buy five years ago for $2300.
There are budget machines well below the $1000 mark. Acer, HP, Apple and Toshiba all make a budget line of laptops that can do the trick for portability at a price.

If you are a college student, be sure to check your bookstore for the student discount; usually around 15%. You might consider buying your software from a deeply discounted education site like journeyed.com. Also browse the student or education section on a computer manufacturer's website. You may have to prove you are actually enrolled, but for 15%, it's worth showing your student ID. I suggest starting at HP's education program or Apple's Education program.

Size is a huge factor in the purchase of a laptop. If you are a road-warrior, you probably want a light-weight machine with plenty of battery life. If you want a desktop-replacement system that lives at home or in the office get a laptop that has a fast processor, many components, and a large screen size; all factors which increase the laptop's weight.
The more you want from your laptop, the bigger and heavier it will be. I travel a ton and I have found the best weight to features equation is about a 4.5 pound machine.

There are a myriad of form factors and flavors of laptops these days. Tablet PCs allow you to use a stylus to write on the screen. Tablets are perfect for computing while standing up or for use in the classroom actually taking notes. Buying a laptop with Microsoft's Media Center Operating System affords you all the features of a regular laptop, but the OS also acts like a TiVo: recording video and storing it on the hard drive. You can plug your laptop in at home and watch your recorded shows on the airplane. Companies like Alienware make gaming specific laptops, and Apple's MacBook Pros come with enough power and style to tempt anyone into switching over.
This entry is just meant to wet your whistle if you are just starting the process of buying a laptop. Here are a few guides to help you dive in deeper once you've answered my three questions about price, size and use.