A gun could get quite expansive, but I’ve never met any occurrence of buying one for a gun-high amount of fur in my research, though rates varied following offer and demand.

For example, during the XVIth and early XVIIth century, the Innus would have a quasi-monopoly over Tadoussac, lowering the prices, as the Iroquoians would be cutting the Saint Lawrence near modern day Quebec. When the French built Quebec in 1608, they undermined the Innu monopoly. Plus, Champlain got the exclusivity of trade from the King and it got even worse after the middle of the century with the foundation of Trois Rivières, Montreal and later Michilimackinac. The Wendat, which were controlling the northern fur trade were wiped out by disease and war in 1648, leaving the French as the main commercial intermediary in the whole Great Lakes and Saint-Lawrence area, raising the costs of a gun consequently.

But, still, at the cutthroat rates under French monopoly, the prices were nowhere near a gun-high amount. It would have taken maybe two or three winters to gather anyway and, as fur-trade expanded, trappers were relying more and more on European food sources like flour, fat and sugar, which they also paid in furs. It wouldn’t have made sense to stop eating for three years in order to buy a new gun.

Delâge quotes a royal document as an example of the fixed rates under the Compagnie de la Nouvelle-France in 1663, and the cost - in that monopoly - was six prime pelts for a gun, nowhere near the maybe 40-50 (?) it would take to fill its height.

I find an even higher rate in the XIXth Century under the CNO (and the CNO has stayed in Cree memory for their starvation-inducing rates). But the price was at 11 prime beavers for a gun.

I am still intrigued by the origin of this legend. I find some occurrences in the XIXth century and I vaguely remember finding something about the Netherlanders asking too much for guns (as a commercial argument : don’t go trade in New Holland, they pay badly), but I will look further into it and find a proper response for this part of the question.

Denys Delâge, "La traite des pelletries aux XVIIe et XVIIIe siècles", Cahier des Dix, Number 70, 2016, [link]

I will also say, as another Canadian, this is the first time I'm hearing of this myth so I wonder if its a regional bit of folklore? I went to grade school in Alberta, university in Manitoba, and my dad's a professor whose focus for his MA was the fur trade in Manitoba. It'd be interesting to know if this is a widespread legend.

I think something to note about here (and it's something you point out, when you say you think Indigenous traders would have caught on) is this is a myth that also relies on assuming the [ETA: Indigenous] traders were, let's say, simple and naïve (even if they weren't familiar with English or French measurements, which they likely would be due to the closeness of that economic relationship, they would have their own systems of measurement to compare to; people could compare gun lengths). Even though this sounds like a legend that was maybe started to demonstrate how settlers swindled Indigenous peoples, it does so by playing into some negative stereotypes.

Other replies are already answering this very well, it looks like, but I do want to say I came across a link, as it pertains to the Hudson Bay Company, that has a chart that lists the price of goods that a Fort sold compared and what how many "made beaver" pelts that would cost ("made beaver" referring to "prime beaver pelt" which other furs would be valued against). But this quote in particular stood out:

The Natives traded for guns of different lengths. The 3-foot gun was used mainly for waterfowl and in heavily forested areas where game could be shot at close range. The 4-foot gun was more accurate and suitable for open spaces. In addition, the 4-foot gun could play a role in warfare.

So it's important to note here that the Indigenous populations not only knew different lengths of guns existed, they would have likely been buying guns with these differences in mind in terms of their utility.

It also simplifies the economic realities of the fur trade. There were French and English traders and for much of Canada, where the English fur trade is concerned, there was the North West Company and Hudson Bay Company -- surely the traders, being in competition with others, might be inclined to barter in cases where Indigenous traders would have options:

This was barter trade in that no currency was used; and although the official prices implied certain rates of exchange between furs and goods, Hudson’s Bay Company factors were encouraged to trade at rates more favorable to the Company. The actual rates, however, depended on market conditions in Europe and, most importantly, the extent of French competition in Canada. Figure 3 illustrates the rise in the price of furs at York Factory and Fort Albany in response to higher beaver prices in London and Paris, as well as to a greater French presence in the region (Carlos and Lewis, 1999). The increase in price also reflects the bargaining ability of Native traders during periods of direct competition between the English and French and later the Hudson’s Bay Company and the Northwest Company. At such times, the Native traders would play both parties off against each other (Ray and Freeman, 1978).

ETA: I will say that I asked my dad about this today, I was curious if he's heard of this myth and it was new to him. What he did mention was that a real issue was the use of "made beaver" as a standard as it's an inherently subjective category, and this intentionally subjective category was clearly to the benefit of those running forts (and the companies could change the standard) while also dealing lower quality goods relative to the price charged; he mentioned Arthur Ray's Short Change as a source.