NB = Narrowband - most commonly Hydrogen Alpha, Oxygen iii and Sulphur ii filters are used to great effect.
See:
http://www.narrowbandimaging.com/home_page.htm
The narrow bandpass gives great contrast and detail - makes you immune to light pollution and even lets you do AP under the full moon. Downsides are only really good for nebulae, colours are false (though you can use blending to get a natural look) and it makes your exposure times longer...
The reason I asked is to understand how many filters you might need to load in the filterwheel (it is a pain to have to swap filters depending on conditions/target).
The QHY range would seem a good match to your budget the 9 perhaps (you might get the filters and wheel inside the budget also if you go that way) - but any of the mono 8300 sensor based cameras should be worth a look.