You need to consider three things when purchasing a good dripper:
1- Airflow control
2- Three posts
3- Topcap design
These three things determine the quality of the vape and ease of use, in my opinion if your on a medium budget the Crown RDA hits all 3 points, fantastic RDA and my personal favourite currently.
Cheaper alternatives wont have as good build materials but are perfectly fine, for example the Omega RDA, or the Trident V2 RDA.
Cheaper ones still won’t have adjustable airflow, which means having to drill them out your self, they may also have poor conductivity, for example, the IGO-W.
To use a Dripper you will need a mechanical mod with a good battery, although you could, use the itaste VV, you will be seriously restricting your vape as the its not designed to be used with rebuildable atomizers! And won’t work at certain resistances.
A good RBA will cost £30 plus, many people are now purchasing direct from china, simple reason: price! Now from experience i can tell you this is not necessarily good idea, although china has caught up in sorts to high end mod makers, the time and effort it takes to make a well machined unit is simply too much to sell cheap! A cheap RBA will leak, short out, and cause too many hair tearing issues to even be worth it. For a very reasonable price a well made tank can be purchased, although not as good as high end gear they still work great, they wont leak from places they're not supposed to leak from, they wont break easily, and they are made from safe materials! The last thing you want to put your juice in is a brass plated cheap RBA! Although you may save money in the short term, you;ll end up buying another every few weeks, as they don't last!
A basic mechanical mod will cost you around £25, remember the cheaper you go the worse the materials and build quality will get.
1- Adjustable pins
2- Materials used in contacts (silver plated very good, copper good, brass ok, steel rubbish)
3- Machine quality.
These three points will determine the vape quality you can get, out cheapest mech mod, hits all three points, the more expensive you go the better the machining and quality of build meaning a longer lasting unit ( this is not necessarily true for all expensive high end mods, some are not very good research is key!)