Price is in €, not $/£. English language is used as international language, not to designate anglosphere countries as the target market.

  ['en-US','en-IE','en-GB','en-ZA','en-AU','de-DE','es-ES','nl-NL','pl-PL','ru-RU','cs-CZ','sv-SE','fi-FI','lv-LV']
    .forEach(lang => console.log(lang + ': ' + new Number('1.23').toLocaleString(lang)))
  en-US: 1.23
  en-IE: 1.23
  en-GB: 1.23
  en-ZA: 1,23
  en-AU: 1.23
  de-DE: 1,23
  es-ES: 1,23
  nl-NL: 1,23
  pl-PL: 1,23
  ru-RU: 1,23
  cs-CZ: 1,23
  sv-SE: 1,23
  fi-FI: 1,23
  lv-LV: 1,23

Apologies, I must be missing your point. If the page is localized to/written in English as this page is, numbers should follow the English style of number formatting. There is an overwhelming consensus amongst English locales in how to format this number.

My goal isn't to be prescriptive, it's to reduce the likelihood confusion on the part of the reader. Writing in English then formatting numbers in a continental style is more confusing than writing in English and formatting numbers in the English style (obscure English locales with relatively minuscule numbers of users notwithstanding).

I've lived in Germany so it was easy for me to understand that €4,00 probably meant €4.00 given the context, but if I were unfamiliar with this style of formatting (as many American's may be), I might reasonably assume that the error was not a comma where there should be a dot, but a missing 0, making the cost €4,000/mo.

If it's your view that writing in English & formatting numbers non-English is more clear than matching the formatting to the language, I'm very interested in hearing that argument.

EDIT: I understand what you mean better now, you're figuring "Price in Euro = European target market." I didn't get that–I assumed the vendor is open to markets worldwide and charges in Euros because that's the vendor's home currency.

better use the user's browser locale + Intl to format the currency for you :)

Easier and better to just omit the decimal part: €4 per user per month.