Re: Have to ask...
It varies depending on if you are vaping mouth to lung (MTL) or a direct lung inhale (DLI), and on the resistance and wattage of the atomizer.
MTL: Less intense hit, physically more like smoking a cigarette but less satisfying. Less complex and/or cheaper/older kit can do MTL. Can use liquids that are 40% to 100% PG. 18-24mg is strong, 12-15mg average, 0-6mg weak.
DLI: Intense cloud hit, physically like hitting a bong or shisha, lots of flavour. More modern kit required, usually using sub 1 Ohm atomizers. Liquid should be 70%+ VG. Lots of nicotine per hit, so 6mg is strong, 3mg average, 0-3mg weak - I know people who buy 0mg and 3mg and mix them in varying proportions..
The cheaper kits will only really allow you to do MTL. For sub-ohm vaping, you will need in excess of 20W, which none of the cheap stuff will cope with. Sub-ohm is where all the development is at these days, you can get mods (the battery + electronics portion) which can drive coils at specific temperatures - one of the reasons why a coil will wear out is if it gets too hot and burns the cotton that it is wrapped around.
With sub-ohm kits, you can do MTL or DLI. You have a mod, a tank and a coil. Some mods have replaceable batteries, which is handy as you can carry several with you on a trip. The tank needs cleaning occasionally, but will last a long time. Coils need to be replaced every few weeks, and cost about £10 for 5. You will use more liquid with a sub-ohm/cloud setup. You can also put on a MTL tank on a sub-ohm mod and dial the wattage down (and your battery will last for many many days).
With cheaper kits, you have a battery, and either a tank and coil, or a "clearomizer", or a cartridge. A clearomizer is a combination tank and coil, when the coil is no good you throw it all away and put on a new clearomizer. A cartridge is a proprietary pre-filled and non-refillable clearomizer (aka "a rip off").
Having started on the cheap kits, my recommendation is to not bother with it. It's unsatisfying, clearomizers tend to leak because they are disposable and everything wears out a lot quicker. I use a KangerTech topbox mini sub-ohm, it costs around £45, you can refill without disassembly, it takes standard 18650 batteries, coils last weeks and I've not had a tank leak on me yet.
Liquids: VG is thicker, and less likely to leak through the cotton in the coil. Some people get irritated throats from high PG content liquids. Liquids with more than 40% VG won't vape well in non sub-ohm/high wattage kit. Almost all the cheap juices you see at the newsagent are 100% PG. Pre-mixed high VG juice can tend to be more expensive, they are more at the premium end of the market and have more complex flavourings - pure VG itself is no more expensive than PG. In fact, by raw cost the most expensive part of the liquid is the flavourings.
For price comparison, I made a 6L batch of 100% VG juice (5L VG, 500ml 7.2% VG nicotine base, 500 ml assorted flavourings) for ~£100, or 50p per 30 ml. Pre-mixed high VG juices are between £15-£30 per 30 ml. In my home made juices, cost wise 10% is VG/PG, 30% is the nicotine, 60% is flavourings