I don't know how to say "how much" in that context, but "this rice" is ez a rizs.
It would be "Mennyibe kerül ez a rizs?" Don't try to translate it literally.
Mine too and it is absolutely correct. Just think about a situation in which a person asks another about the amount of the rice. Like the asker is not sure about the grams and whatever, but relies on the asked person, who may know it better. Mennyi rizs ez? can mean what is the amount/weight of this bowl of rice.
I'm just saying that it is totally uncommon (I've never used this sentence in my life and neither my relatives nor my friends [yes, i asked them all]). If they still want to teach the sentence "Mennyi ... ez?", they should use something else, which is common.
If I'm not wrong, we still haven't been introdfuced many other uncountable words other than rizs, víz, bor and páprikast