Duty, under nafta free trade (whatever it is), means that pinball machines made in the US are duty free. I'm assuming you are aware of duty costs for products purchased in the states, such as bringing back merchandise and alcohol and such. Anyway, no matter how much, pins are duty free.
As for taxes, you are required to pay our HST for any product purchased in the states. This means that on $3,500, you would be responsible for $455. You ***WILL**** need a invoice of sale from the seller with their contact number. Without this, you can get into a ton of crap at the border, and anything is possible (not believing your story, charging more...).
Now having said that, with bringing anything back, they could either waive you through, or bring you in, charge you the taxes and make you tear the machine down to search.
I went down with a group to Allentown this year. I returned with a LOT of parts, LEDs, and we had a trailer with I think 5 machines purchased. We picked a line at the border where every car was going off for a "secondary" (off for a search), and given our trailer, we couldn't fathom that we were making it out easy. Once we spoke to the border guard, he found it amusing that we were at a pinball show, looked briefly at the full amount of receipts we had, which were in order, and waived us through, even though I was about 4 bottles of scotch over the duty free limit.
Whatever you do, just be honest and straightforward. I have heard from many collectors that have had very bad experiences due to trying to knock the paid price down when declaring and lying. They have to know and have price values for EVERYTHING.