They’re just likely going off of past experience. The only bi-metal mystery was in the previous round of mystery boxes, and that was the 2016 Superstar; even then, it wasn’t the main mystery box, as it was only $50 if you bought one of the other two main mystery boxes, otherwise it was $100. I suppose one other mystery box in the past had a bi-metal, but that was a “collector’s edition” box that was $200 and had $450 worth of stuff including a Space Cowboy.
The general pricing of mystery boxes is $40-50, usually with $80-100 of value.
As for my guess, I’d say Nightshadow was probably correct, I think it will just be a new mono-metal with maybe an older mono-metal or some other throw, as otherwise YYF would be saturating the bi-metal market. With the Edge, Ex Machina, and Space Cadet all releasing not too long ago, I doubt they’d release another bi-metal.
I agree with this, to an extent but I will tell you why I personally like mystery boxes so much.
First it allows me to try new yoyos for really cheap.
Second since they are really cheap, you can work with them to get some good money back. Here is a sequence of a few trades I did.
I used a mystery box yoyo that I spent $25 on (2 yoyos for $50). I traded that for two yoyos, a plastic and a metal. I traded the metal for another yyf metal. That metal was traded for two more metals, a b grade and a dream yoyo of mine. As well as a cheap Kendama. I sold that b grade for $27 and then kept the rest. That $25 dollar yoyo ended up getting me $27, 1 cheap plastic yoyo, a kendama, and a metal yoyo that I had wanted for a long time, I did have to pay for shipping on all of this but that is very minor (probably less than $12).
So yes, I will be probably buying a mystery box since they are yoyos on sale, but in the end, whether I like the initial yoyo I obtain or not, I will be exceptionally happy with what I have.