Nepenthes northiana is a lowlander and best grown that way. I have two plants. One of them I tried to grow outdoors with bad results. Although the plant survived, it really took a beating. Plenty of leaf curl and tendril burn. I'm growing them indoors and they are doing much better as a windowsill specimen. As of late, the plants are doing better and adjusting to windowsill conditions. This species has been around a while and is very beautiful. I've heard that it can be finicky with humidity and is not a real easy lowlander to grow in optimal conditions. There's plenty of great photos of this plant all over the internet and are worth a look. This plant is proving to be one of the most difficult species for me to adjust to my conditions of growing similar to that of N. bicalcarata. Under normal lowland, greenhouse conditions I haven't heard too many problems with this species.
Cultivation: difficult to impossible as a highlander;
difficult as a windowsill plant, easier as a lowlander in normal greenhouse
Market availability: size 2, to size 3; fairly common
Species variability: none that I'm aware of other than locality differences
$ / size: inexpensive to moderate; $17.00 and up
Cuttings: none available for trade