Late to the party, but... Having lived in Germany for a few months now, I've noticed - in the East at least - that when people talk about cheap quality they say "Billig" but in talking about prices they say "Günstig". 'Gunstig' is cognate with favourable, but this is how the two words are used.
It's just... what if someone doesn't pick up on the implication? I kind of had to to get the answer right, but some young hooligan might not understand. They might not see that it is meant to generalize the merchandise at the supermarket under the name of the supermarket itself.
30 years ago, even i was taking high school German, my teacher was very particular about using "sein" in sentences. He always defaulted to "sein" being a statement of equivalence rather than an introduction to an adjective. You aren't hungry, "Sie sind hungrig." Rather, you have hunger, "Sie haben Hunger." That could have just been his way of getting students to think more like Germans.
If you ask someone standing in front of the supermarket "Ist der Supermarkt billig - is the supermarket cheap" they would get that you mean the merchandise, not the supermarket itself. But if you ask if it is affordable - erschwinglich that sounds like you would like to buy the property.