Okay, so 1/3rd of the monthly wage means 1/3rd of a month, meaning 26 hours of 4*40 hour weeks.

Assuming there's no other overhead an independent contractor needs to take care of in Italy (in the US the big one would be health insurance, but you live in a civilized country. On the other hand, your civilized country probably all takes reasonable paid vacation, and an independent contractor would probably want to charge enough to do the same. And there are other reasonable reasons contractor hourly rates tend to be higher than pro-rated full-time salaries, things included with a typical full-time job above the salary itself that a contractor has to pay for or spend time on themselves).

But okay, let's say 26 hours.

How many junior developers can take an idea, probably somewhat vague and poorly thought out, and turn it into something useful in 26 hours?

I'd say if they can, they probably ought not to be considered a 'junior' developer!

It is not.

It's 1/3rd the net monthly wage, i.e. probably less than 1/6th of the money you pay to have junior developer actually working for you for one month, once you factor in income tax, company tax, social insurance, sick time, vacation time and national holidays.

I wouldn't expect a junior to be able to manage the whole stack and get stuff on a server. Write an application probably, but it took me a few years before I could manage everything.