While MOST rubles / poltinas that feature Ionna III is quite pricey, as it is probably the only time during the Imperial Russian era such a baby was featured on coins (even so, babies aren't common in recent Russian commemorative coins!)
And hence, the cheapest that doesn't feature Ioann's portrait is the denga, which I think is pretty difficult to find.
Here is one currently on auction at the moment:
Link
And now we shall compare to one that is currently offered by Alexsander Basok.

I think you can see the difference in the location of the number "4" as well as the sharpness of the letter "G" in denga.
Not too sure which is the rare variety, but I don't think I have come across a lot of 1741 dengas either, making it difficult to make a realistic idea of how uncommon they might be.
I know I don't have one.