Some of them are made in different locations, making production cheaper or more expensive. Also, often the high-end atomizers are not only made in a place where manufacturing is expensive, but they are double checked to make sure all machining tolerances are tight... sometimes they power wash any left over machining oils too. And they come in a nicer box. And they have a real serial number.
But a $30 1:1 clone will often look and perform exactly the same as a $150 authentic RDA. A $5 clone probably has much cheaper screws and no QC, making for a good chance of breaking... you will probably get $5-worth of use out of it though.
As for the differences between different RDAs, they are the size and placement of airflow and the way the
coils are mounted. Also the size of the chamber plays a factor. Sometimes there are innovative top caps too, but the main distinguishing factors are airflow, coil placement, and chamber size.
IMO, a prefect RDA is anything that allows a lot of airflow, has post holes that accept all kinds of builds using lower gauges of wire, and has a wide-bore top cap or the ability to switch out top caps. But not everyone builds as often as I do... I do this as a hobby. So some people might prefer something that is more similar to a cigarette.
