Size and Weight - Weight is a pretty obvious one because a drone under 0.55 pounds (250g) allows you to skip registration in most countries, But what about the size? Well, the smaller the drone, the less wind resistance it usually has. This also applies to battery life - smaller drones have shorter flight times.
Flying a small quadcopter indoors can be fun; however, if you take it outdoors and the wind picks up, you could quickly lose control. So be sure to consider your environment before flying (and buying).
Cost - How good can a drone get, in fact? Short answer: as good as the price allows. If you're looking for a drone with 4K cameras, stabilization, GPS and brushless motors but can't afford to break the bank, there are certainly a few trade-offs that come with it.
However, keep in mind that some overpriced drones out there (quite a lot, actually) will trick people who don't know what they should look for in a drone specifically.
Spare Parts and Popularity - Yes, popularity is essential in some way because it will allow you to find spare parts easier, new or used. You can also buy the drone second-hand directly if it's popular enough, saving you some money.
Brushless Motors - Brushed motors can be found on cheaper toy drones that work as beginner learners but are inferior to their brushless counterparts. The brushless motor is indeed more expensive but much more durable and powerful. That's why these are used in larger drones or more premium ones (usually beyond the $100 price point).
GPS and flight modes - with drones this small, you usually don't get very advanced GPS flight modes, but you can certainly get good GPS and, like in the case of the DJI Mini 2, even other satellite systems like GLONASS and GALILEO.
Camera - I advise you not to get a camera drone if it's under $100 because the footage you get out of it is usually pretty crap. The drones on my list have decent cameras, but usually, it's best to wait until you have at least $200 to invest in a drone so you can get a higher-quality rig and maybe even image stabilization.
Some rules for drones under 250g
There's obviously the advantage of not registering your drone if it's under a certain weight, but are there still other limitations?
Can you fly a drone under 250g anywhere?
No matter the size, all drones must abide by the same flying regulations. This means you are still not permitted to fly close to an airport or military base, even if your drone is under 250g. Most of the time, you can find these areas marked on a map like this.
Having said that, there are also limitations on how far you can fly away from crowds, property and such; this all depends on the specific country's regulation.
Which is the cheapest 4K drone?
If you're looking for the cheapest 4K drone, that would be the Eachine Ex5. At the same time, the camera quality on this drone doesn't feel as advertised. So, the cheapest actual 4K drone remains the DJI Mini 2.
What is the lightest drone?
The lightest drone is the Cheerson CX10, a tiny quadcopter that weighs around 12g. This drone can fit in the palm of your hand, but it's quick, fun and excellent for indoor flight.
Although buying a drone under 250g means you don't have to register it, you still need to be careful of your surroundings and where you fly it.