Preposition по in the context of price implies that it's the price for each and every item individually.
But although the question почём? uses Prep. case, in the answer numeral is inflected in Nominal.
This is a remnant of an old paradigm. In the past, inflection of some numerals (or even all except those ending with 1-4, not sure about that) in the actual Prepositional case was common, eg.
<Монеты> приносить на Монетной Двор, которым за тѣ монеты наддавано
будет, сверх настоящаго, по пяти копѣек на рубль.
Словарь русского языка XVIII в.
даютъ двѣ кади меду по шести пудовъ, а хмелю двѣ бочки
Акты, относящіеся к исторіи западной Россіи (1846) vol.1, p. 103, line 18 from the top
In fact even today Prepositional case is still occasionally applied to the numeral тысяча - по тысячи.
Участники Великой Отечественной войны получат по 5 тыс. рублей,
труженики тыла по тысячи рублей.
newslab.ru March 16, 2010
В Забайкальском крае более 1 тысячи семей с детьми, находящимися в
трудной жизненной ситуации, получат по тысячи рублей из бюджета
zabinfo.ru Feb 11, 2014
оба угнали из СССР по тысячи лошадей
Быков Н. А., Казачья трагедия (1940-1945-й г.г.). — Нью-Йорк: изд. автора, 1959.
I don't think it's just a mistake because there're far too many examples, although in Google results its occurrence is still significantly outnumbered by instances of the Dative case по тысяче.
по governs bona-fide Prepositional in the meaning after, upon
по прибытии/приезде/прилёте - upon arrival
по окончании - upon completion
However i subjectively feel the urge to use Dative (по прибытиЮ etc.), it feels more convenient, so it could indicate that in this use Prepositional is a paradigm which is gradually obsolesces.
Moreover i think that conceptually по governs Prepositional in most cases, despite the fact of inflexion assuming the form of Dative (maybe it's a heterodox point of view).
For instance it does so when it acts as Locative in which action is applied to or takes place on the surface of an object:
идти по мосту
ходить по двору
стучать по столу
гладить по голове
плыть по воде
ехать по суше/земле
бить по зубам
The ending of feminine nouns is identical with the standard ending of Prepositional, eg
о чём? - o голове/о воде/о суше/о земле
по чём? - по голове/по воде/по суше/по земле
To these phrases both questions по чём? and по чему? apply, however the former already sounds dated or vernacular.
Also in the sense of by means of
связываться по сети
говорить по телефону
показывать по телевизору
In Russian the difference between these special instances (in my opinion) of Prepositional on the one hand and Dative on the other has blurred, but in Ukrainian, in particular, in locative function, governance of Prepositional with по is maintained for plural forms.