I am very new in email marketing. What would be an usual "price per
sent email" for a legal email campaign, if I do not own any mailing
list.
Thanks

I am not a Google Answers Researcher

I'd be curious to know whom you wish to target if you don't own a
mailing list. Your cost per email can be over $100 in court costs if
what you're sending is considered spam and based upon a list of people
who didn't opt-in to your list...

OTOH, your cost per email is negligible for the action of sending the
emails with a mass emailer. (The cost per send generally goes down as
the quantity goes up, if you purchase a mass emailing program).

In my question "price per sent email" includes legal optin adress (rental?) fee

Thank you for your question.  
 
However, I believe that to answer it well, your question will require
more time and effort than the average amount of time and effort
associated with this price. Here is a link to guidelines about pricing
your question, in the pricing guide:
https://answers.google.com/answers/pricing.html

Costs for rental and sitribution of legitmate opt-in (permission
based) email lists can vary widely based on a number of factors. But
in general you should expect to pay between $100 to $400+ per 1,000
emails. Costs vary based on the quality of the list, competitive lists
availabel on the market, demand for the lists, whethter they are B2B
or B2C, if you order the lists to be segmented, etc.

I recommend going through a list broker rather than direct to list
owners - the cost is the same to you, but you have the additional
value of a list broker (good ones) providing insight into the track
record of the lists, etc. Never fall for the sales pitch some push to
"buy" lists. No legitmate list owner sells their list, rent yes, sell
no. You will not receive the lists - usually a list manager (company
that manages the lists for a list owner) will distribute your email to
the list. Most list owners require that you rent a minimum of 5,000
addresses. I recommend that you rent a couple of lists at the minimum
level and then conduct A/B split tests to determine which lists and
creative approach (offers, design, copy, etc.) produces the best
results.