Hi Fabrice, welcome to the group!
This bass might be considered to be expensive by some who have observed that similar Series I basses have sold for less on Ebay in the US.
Generally, prices for similar basses will be higher in a retail store than in a private sale, and higher in a private sale than in an Ebay auction.
Generally, prices for similar basses will be higher in Europe than in the US.
Time is also a factor. If this bass meets all the requirements that you are looking for, and if you don't want to wait around for another bass like it to show up on Ebay, then you will be willing to pay more than others who are willing to wait.
With this specific bass, as Frank has pointed out, there appears to be some question as to whether this is a long scale (34) bass with a small standard body. While there are one or two members of our group who prefer long scale basses with small standard bodies, most folks find that they do not balance well. That fact, when known, should reduce the number of potential buyers, and thus the price. However, long scale Series I basses with small standard bodies are rare, and thus those who are specifically looking for such a bass are probably prepared to pay a premium.
The seller's listing, as Frank pointed out, did not help clarify the scale lenth of the bass, and appears to actually obfuscate the issue. The scale length can be determined by taking an accurate measurement from the nut to the 12th fret and multiplying the result by 2. Most Alembics with small standard bodies are short scale (30 3/4).
You are certainly correct that the listed price of this bass, even with the electronics upgrade, is substantially less than the price of a new Series I. And I would guess that the seller, if patient, may get this price if a buyer looking for this particular bass, or someone looking to buy now rather than later, sees the listing. But I also think that other buyers can reasonably put this bass in the context of other similar listings and their own set of parameters and find the listing price to be expensive.