Tripods are actually a much more complex device than most people realize. There are three main things I can think of that will make a tripod cost more money (that are worth paying for), materials, weight rating and head type.
The material mostly contributes to the weight of the tripod. Materials like carbon fiber are super light, but make the tripod much more expensive. If you need portability though, it may be worth it.
Weight rating is how much weight the tripod and/or head can support. Also be sure to note that while your camera may only weight x amount, when you have it tilted at an extreme angle, there is going to be additional force on the head due to the lever action of the tilt. You need to have sufficient weight rating for your camera and lens and associated gear. Note that the weight of the head must be factored in when calculating the weight that you need the legs to hold (most high end tripods are purchased as a separate set of legs and a separate head that you put together).
Type of head is the actual technique used to lock the tripod down. The simplest of all would be a fixed head which wouldn't allow movement. There are then ball heads which are simply a ball in a socket that can squeeze the ball to grip it. These don't provide independent control of axis, but for photography are often sufficient since freedom of movement in setting up the shot is key, but you don't need to follow with a locked axis. Fluid heads are one of the more elaborate heads and involve different axis of motion that are designed to glide on plates in the head. These allow for smooth motion on multiple fixed axis and you can lock those axis down if you only need to control the pan or the tilt (or have varying degrees of resistance. These are some of the strongest heads, but also much MUCH more expensive than ball heads. (In the $100+ range even for a basic model, with $200+ being likely for something that can hold 7 or 8 pounds and that's just for the head.)
To the second part of the question, are they worth it? Absolutely. They are far more secure and will allow better control, more stability and smoother motion if you do any video. It's tricky to decide exactly what your needs are, but certainly start by weighing your tripod and deciding if you want to be able to use it for video. Then decide how much mobility you want and buy accordingly. Sites like B&H also offer really nice filters for helping you narrow in on a tripod that will fit your needs. Personally, I'm a big fan of Bogen and Manfrotto, but there are many other great manufacturers as well.