Since the OPs question made me curious, I recorded my catches using the rare fish lure over the last few days. Keep in mind that the sample probably is still too small to be statistically significant, but it does show a rough trend:
Originally Posted by Mouset
(obviously, "lures used" needs to be multiplied by 5 to give you total fish caught)
Zone Lures Used Rares Rare Ratio
Scarwood Reach 11 17 30.91%
Scarlet Gorge 10 11 22.00%
Droughtlands 11 19 34.55%
Moonshade H. 9 9 20.00%
It takes me just under 2 minutes to use each rare fish lure. So just looking at Droughtlands and Moonshade (since this is where you get the fish or the rare fish lure), I used 20 lures in total (around 40 minutes of fishing), and caught mats for 36 more lures. Let's be cautious and say you can get 50 rare fish lures per hour that way. That's gotta be faster than fishing them up normally
I keep about two thirds of them for getting more, and use the rest on the fish for the timed magnetic lure. That's why Scarwood and Scarlet Gorge are in there as well, needless to say.
Although you have to keep in mind that you won't get artifacts that way, so if that's what you're mainly going after, you need to factor in the time spent fishing for the rare fish lure to work out if it's worth it. Rule of thumb: if you can manufacture 3 or more timed magnetic lures per hour, you should come out ahead (I won't bother with putting down the math, since it's only a rough estimate I worked out in my head, but feel free to double-check).