Honesty is always the best policy but if you're worried about it you could assume a cost of $500 which may be reasonable as well. It would still be over the $400 you would be allowed to bring after this trip but given the strength in the CAD it would be only about $75 over the limit. Also, you could just put the computer and charger in your bag and not say anything, it is not uncommon for people to bring laptops back and forth, just make sure you have done the original set-up before you get to the airport.
If it was me, I would declare it and I find that they are usually pretty nice to you if you declare. I often come back with a bottle of booze after being gone a night and declare the 20-30 dollars I spent and they often kick me without paying anything so I think they appreciate the honesty. Also, if you travel a lot, you don't want to get in their bad books, particularly if you use Nexxus or Canpass.