Nicolas, you have scratched the surface of a topic that obviously has undertones that some of us 'old guys' still need to debate over a beverage.
I am not going to weigh in on the more difficult conversation, but I am willing to suggest the following:
Michael's first response under the title 'Not useful, but', was actually a very good answer to your initial question. In essence, he was saying that, if you are willing to pay $400 for that tunic with the information you have, then it is worth $400 to you, but it may not be worth $400 to other people.
Over the years, I have added about 35 war time Canadian battle dress tunics to my collection, a couple I purchased in the 1970s, and the others acquired over the intervening years. The last one I purchased was 3 years ago, and it was an attributed to a full colonel; a well known war-time officer, and I confirmed the provenance with the family, and I only purchased it because it was named to an officer who I was familiar with. I bought it out of a very well known collection, whose owner is known for his honesty and integrity. I paid $300.00, and it will remain in my collection for a long time.
I have seen lots of Canadian battle dress tunics for sale on the internet and elsewhere, and in my opinion, $400 is simply too much unless the officer who owned it is of personal interest to you, and you have direct evidence that it was his tunic. I should add that I have also passed up a lot of battle dress tunics that didn't meet my careful criteria.
The best advice I can give you is, don't buy militaria expecting to make a profit on it. There are many professionals (a few of whom are regular contributors on this site), who would have beat you to the bargain, if it was really a bargain.
Collect only for the 'love of the sport', and be careful with your money.