See the following "simple" memorandum:
http://www.cbsa-asfc.gc.ca/publications/dm-md/d8/d8-2-10-eng.html or talk to a customs broker.
Lots of what follows is more theoretical than practical. The value of the camera and the value of the repairs may determine everything.
There will be an official tariff item number that applies to items being returned to Canada after being temporarily exported for the purpose of repair.
And there will be a US tariff item number that applies to items being imported into the US temporarily for the purpose of repair and subsequent return to Canada.
In some cases (think huge pieces of very expensive machinery) US Customs could require either:
a) payment of duty, with subsequent application for refund accompanied by proof of re-export; or
b) posting of a customs bond to secure duties that would otherwise be due.
Unless the camera is exceptionally valuable, and there is in fact a duty rate that applies to it, this shouldn't apply to your situation.
With respect to Canada Customs, THEORETICALLY you need to be prepared to show that the camera was in Canada with you before you temporarily exported it and that, in some cases, it was originally imported properly into Canada. Then you need to show that it was, indeed, temporarily in the US.
Canada Customs would certainly be entitled to levy duties and applicable taxes on any additions or improvements to the camera, including any parts added. For some work, where it is difficult to separate the value of the work done and the parts added/replaced, you could end up paying on the labour too.
Whether or not they would do so, is more a case of practicality.
Another warning, and I doubt that this applies to cameras, but it does apply to cars/trucks. If you send a car to the US to be serviced, or drive it to the US for that purpose, the entire value of the car becomes dutiable - not just the value of the repair.
Finally, I do not recommend listing an artificially low value on any documentation. If you do, and you also insure the shipment, guess what value the shipping insurers are going to give you if you make a claim.