by chance were you reading the monitor guide by manofchalk and i? that is stickied at the top of perepherals. it is chock full of good information but really does not go into pricing since values do change over time.
you asked about what the bad points are with tn.. here is a short photo lesson.
typically bad viewing angle
6bit panel uses dithering to reproduce colors instead of natively supporting them like 8 or 10 bit ips
the good points of tn are the response time, refresh rate and price. response time is the time it takes for the monitor to display an image on the screen after it receives a signal. currently tn is the only monitor capable of 120/144 refresh rates.
a higher refresh rate means a smoother video flow however realize that this takes alot more processing. some new high end games can barely run at 60fps on a single card. to get higher fps you would have to drop settings down lower which is why some people use sli or crossfire at more expense. realize also that to keep using high settings and keep high fps you will need to update your gpu more regularly or drop settings as time progresses. granted you dont have to run at high fps on a 120/144hz monitor but why would you buy one if you dont plan on it?
this is why i said what monitor you pick is according to your wants/needs.
tn @ 50/60hz = no high refresh rate, poor viewing angles, poor color accuracy, no high refresh rate but okay if your hardware cant support, very good response times. cheap.
tn @100/120/144hz = same as above but supports high refresh rate. you need good hardware to support this. mid range price.
e-ips @ 50/60hz = no high refresh rate, very good viewing angle, average color accuracy, average response time. mid range prices.
va @ 50/60hz = similar to e-ips.
s/h or p-ips @ 50/60hz = no high refresh rate, very good viewing angle, best color accuracy you can get, slightly high response times, high price. these are the best "image quality" screens you can buy but they are very expensive.
pls @ 50/60hz = similar to above except cheaper prices due to a cheaper manufacturing process over how they make ips panels.
keep in mind that picking a 1200p, 1440p or 1600p over a 1080p panel will also drive prices up.