condomboy is right. That's the golden rule when it comes to selling used guns!
Is it a 9mm or a .45ACP? The .45 is rarer and more desirable for collectors. I wouldn't have even thought that there were any .45's in Canada, but I spend too much time chatting with "Nemo" (Roger) at my local range to harbour such illusions any longer. If it is legal to bring in, somebody will do it, and somebody else will buy what that importer brings in. That's where the price thing comes in.
The prohib ones, like a standard P9S 9mm, the prohib status has slaughtered the value on those. When they come up for sale now they go for South of $1,000 and that trend will only get worse in the future. The very few re-barreled restricted ones (I have one) seem to go for around $1,250 to $1,350.
The compensator ones, also very rare. ARMTAC brought in a few of them last year and I am trying to remember what they sold them for. It was a fair bit. I think about $3,000. But everything goes on the details. If you had something in really pristine condition, with original box, manual, mags, etc. it could go really high. Something that is of little or no interest to collectors, the value will start to go down just as fast.
You have to remember too, most of the P9's are like 1970s era. Most of the ones I have seen actually precede the HK date coding system, which I think starts in 1980. These are guns from 40 years ago! It's not like anybody is going to buy one to compete in IPSC, or as a first gun, or a gun for their wife, etc. They are collector guns. If it is in good condition, collectors will pay for it. If it only has value as a shooter, price probably drops off precipitously.