Sorry about the t/d mistake. I'll get that right eventually!
In English also the perfect/imperfect distinction is not always made clear (unlike in French, for example). To make it clear, we use "used to pay", "were paying" or "would pay". How would "how much were you paying for dresses" come out in Dutch?
Don't worry, a lot of Dutchies make the d/t mistake all the time! :P
I'm not sure, but the difference between the perfect and imperfect is not clear (I think) (You mean the conditional, right? Both perfect and imperfect..).
I guess I would translate that sentence as 'Hoeveel was je aan het betalen voor de jurken?' It sounds incredibly unnatural, though. Maybe I'd rather say 'Hoeveel heb je voor de jurken betaald?/Hoeveel heb je betaald voor de jurken?' ('How much have you paid for the dresses?')
My point was that in some languages the perfect and imperfect are always expressed differently. "j’ai payé" and "je payais" are not interchangeable. English does make the distinction, but not rigidly, by using circumlocutions. Apparently the distinction is not recognised in Dutch.
Did is the auxiliary that needs to be used to form questions in the Simple Past tense whenever the main verb is not to be.
What did you eat?
I atea sandwich.
Did you buy it at the cafeteria?
Yes, I did.
Note that it's also used to make negative statements (also in the Simple Past).
I didn't want a sandwich, though, but that was all I could afford.