It really depends on what you're looking for...some are more expensive then others, so have a nicer finished look (ex: stainless steel endcaps). If you buy cheaper, the tube might be rough and unfinished. Or the end caps might not fit as tight as you would like, meaning you have to be careful to prevent leaks. Another good quality to look for in a tank is having a fill hole, so you can depressurize a tank easily. The big thing is end-cap tightness, to prevent leakage. Otherwise you can get a good tank really cheap.
j-tank from electronicstix: I really like the build quality, and it's great for 20 bucks. The endcaps are ultras snug, and it comes in some cool colours.
Chibi tank from parkesvaping: It's kinda weird putting together and it's definetely unfinished (uneven tube cut, weird wonky "o-rings") but it doesn't leak, and it's 5 bucks. Oh but you need to fill by squeezing a needle around the end-cap which bends the needle if you're not careful.
Both these tanks are fairly cheap, have their strengths and weaknesses, but my vape is suffering from not having a fill-hole with screw to de-pressurize the tank. With the j-tank, I have to take off the drip-tip and, slide it up to get some air in. With the Chibi tank, I have to stick a letter opener in the edge, and it's a little akward. You get what you paid for with either.
The tank I'm interested in looking at next is the mini-sub from mom and pops vapor. It's 12 bucks, looks like a snug fit, the finished product looks pretty good to me, and it has a fill hole with screw.
I think it just depends on what you're looking for. I'm kinda new to tanks, so I'm only familiar with the lower end tanks...Oh and the Smoktech tanks are really okay too for 10 bucks. But the end-caps aren't ultra snug, so you have to be careful when sliding the tank up to fill, and dual coils burn through my juice. It's still not too bad for 10 bucks, if you fiddle with it a bit.