No you can’t use “désormais” or “dorénavant” like “maintenant”.
It is the same for “now” and “from now on”. “Désormais” is used to define the starting point.

When you say “now” (“maintenant”) you mean that it is currently occurring but you don’t know or don’t care when it began, whereas "from now on" (“désormais”) indicates that it just began.

For your sentence it can only make sense with the context:

Ils viennent d’augmenter le prix de la levure. Êtes-vous conscient
du prix qu’il faut désormais payer pour acheter une tranche de pain ?

(They just increased yeast price. Are you conscious of the price we
will have to pay for a slice of bread from now on?)

If you want to substitute something else for “désormais” you can use “à partir de maintenant”.

In English, you would only say “now” because of a commonly stretched usage of that word. It’s a bias. Strictly speaking, what is meant about the price of bread is actually “from now on”, not just “right now”. In other words, it’s saying “now” in English that is incorrect, not the other way around. 🙂

The question being whether it’s OK to use “désormais” – it’s not only OK but in fact the proper word. It is also OK to use “maintenant”, though… just like in English!