How much cost a professional SEO service?
Basically, In France, there are 2 major types of professionals: agencies and freelancers.
Most of the time, an agency will be more expensive than a freelancer. However, some high-level freelance consultants may charge more than a small agency.
You've got it well, there are a lot of diverse prices, starting from 60 euros to more than 150 000 euros per service.
The cost per day may be a good indicator for comparison. The most common rates are between 100 and 1 000 euros per day.
A prestigious agency or a very skilled profile can go up to 1500 euros per day.
In any case, you should compare the number of working days. It is not possible to really follow and supervise a project for 12 months by working only 3 days (between 900 and 3,000 euros then).
Under 12 days of work, please keep in mind before signing that the commitment of the professional is less than 7 hours per month.
Anyways, if your goal is to generate 30,000 euros in turnover through natural referencing by investing less than 3,000 euros, you're probably already on the wrong track.