These are awesome snakes. I have 17 of them and don't feel it's enough 
However, $280 or whatever you wrote is very expensive, although I'm not sure of the Canadian exchange rate. I'd be very sure that it was a captive born individual for that kind of money. Imports in the US sell for about $50.
Regarding your questions.
Gonyosoma seem to do well in a variety of cages. I know breeders who have kept them in 28 qt. rubbermaids that are less than 6" deep and have a floor area of 20" x 14". How in the world they managed to get them back in the boxes is beyond me. These are fast, agressive species that can be difficult to handle. I've worked with a lot of mambas, taipans, cobras etc. and none of them were as slippery as my Gonyosoma.
But I digress. I currently keep mine in boxes that about about 30" long and 12" to 13" tall. I would generally describe this as a minimum size. I offer perches but they don't spend a lot of time on them. This species seems to be facultatively arboreal. Only my most established specimens climb on a regular basis. I try to cover the perches with sprinklings from the bottom of a bag of cypress mulch. That way if they climb at night I'll see tracks in the mulch. It appears some of mine just don't climb.
My biggest adult is over 700 grams and her sheds are 8' long. With the humidity in her cage I'm sure she's considerable smaller than this, however. Regardless, I have a few females that are just over 6' long.
Are they easy to keep... there's a difficult question. The safe answer is yes. I've certainly kept more difficult species, but these are definately tough. I would not describe them as difficult as wild caught Mandarin or 100 Flower Ratsnakes, but they are difficult.
My mortality with imports is about 35%. It's actually hard for me to estimate because a few of my cages were under nasty lake water when my herp room flooded. The snakes survived the accute trama but did die about a month later despite my efforts to keep them going.
However, I have a friend who is able to handpick them from an importer in the states. We are very lucky. Even then we have specimens that seemingly thrive and then go into a bad shed and die.
On the other hand, my one captive born specimen might as well be a different species. He is very hearty and just does not cause me any worry.
Hope this helps.
