There are lots of ski models out there, ranging from $60 to $2000. But what difference does price actually have on the performance you get out of the skis? How does one know that it makes sense to invest into more expensive skis - or possibly when it makes sense to sell your skis for a cheaper model?
Generally, the more expensive a ski is, the more expensive material and manufacturing processes can be used. For example, cheap ski typically has a foam core which is quite soft and is equally soft everywhere. This has the advantage that the ski is more forgiving than a harder ski. However, when you start to ski faster this turns into a disadvantage as general softness makes the ski unstable and imprecise to control. This is where higher priced skis come into play. Using a wooden core and materials like titanal they should be more stiff and precise and due to a more complex construction the stiffness and softness of the ski are at the right place.
Another important aspect is the base of the ski. An expensive base should be sintered and therefore be more durable. It also should take on wax better and therefore go faster.
As always with equipment, there is a diminishing return. I definitely would not go for something that costs 60$ when new as it is impossible to produce a quality ski and still earn money for everyone along the manufacturing chain. On the other hand, the 2000$ ski will definitely not be twice as good as a 1000$ ski. The sweet spot probably is right in the middle. I am currently looking for a new all-mountain ski and there are a lot of interesting and good performance skis in the range of 500 to 700€ real price including factory mounted bindings (recommended selling price can be about 200 higher). A touring setup is a bit more expensive, mainly due to the fact that bindings are more complex and sold separately.
Selling your skis for a cheaper model probably is never worth the effort as all equipment loses a lot of value the moment you walk out of the store and it will be hard to get something as good for a lot less money. The other way round it depends on what you need. After some time, you will outgrow your beginner ski and may need something that is more suitable for higher speeds.
Note that better or worse is not a single dimension. As soon as you leave the range of the cheapest skis you first need to find out what you want to do with them. Race and slalom carvers are for fast and aggressive skiing with the main difference being the turn radius. Those need to be very stiff and need to be stiff at the right place. Freeride skis are made for powder and tend to be a lot softer which is fine off-piste. But a wide ski will perform abysmal on an icy piste. And there are all-mountain skis which are allrounders but will never match the performance of a pure specialist in their area.
Another important point to note is that the boot is as important to your skiing as the ski itself. It should match the type (and therefore stiffness) of the ski. Changing to a new type or quality level of a ski often involves changing the boot as well.