I have found the following article regarding Qué vs. Cuál to be very helpful in understanding the reasons behind when to use one vs. the other: https://www.realfastspanish.com/vocabulary/que-vs-cual
If you say "Qué es el precio" that would be like asking for the definition of "el precio". Although qué is usually used to mean "what" and cuál is usually used to mean "which", there are times when qué is used for the latter and cuál for the former. It can be a very confusing concept. This can help you understand a bit: http://spanish.about.com/cs/grammar/a/que_vs_cual.htm
In a previous exercise we had Que in something like 'Qué talla buscas/quieres/necesitas' from DL ; i assume from advice here that if I invert the sentence to use ser, it would become 'Cuál es la talla que buscas' ? and similarly something for this one like 'Qué precio pagé' where ser is not used?
Yes. It's a contextual thing. "Qué es" is usually used to ask for definitions (e.g. "what is a price?" "it's the amount you pay for something".) So before the word 'es', 'Cuál' is used instead when you're not asking for a definition.
I looked it up, apparently they use cuánto es? For total price, and cuánto cuesta for 1 item. So the question is, do people in Spain or in Latin American countries use Cuánto es? Or cuál es el precio? Are both ok? Or is one preferred over the other depending on where you reside?