I've looked online abit, and I usually run Sandbox games so they can last quite a long time, so I was curious what would the costs for juvenat treatments be?

Not something I'd count in Thrones, at least. Much like capital ships, this is something where you should be looking more into the realm of favours and powerful connections to get an "appointment" with a skilled Magos Biologis. Althouth, given how the Imperium as a whole runs on a barter economy, you could also offer relics in exchange, or - if you're an Inquisitor - simply try to confiscate a sufficiently large shipment of goods to make it attractive for the relevant division of the AdMech.

Inquisitors generally have the right to demand this treatment, anyways, but this being something "on paper" does not necessarily translate to it actually happening in the real world. I'd expect a very entrenched system of backroom deals and silent understandings that would simply see someone who doesn't respect it getting deadlocked by courteously formulated delays or the necessary resources and/or personnel happening to be "unavailable at this time".

For ranking members of the Adepta, I imagine there is a sort of official waiting list that allots a certain number of "slots" to each organisation, leaving it up to the individual Adeptus to distribute them among those deemed worthy. But needless to say, this would also have potential for bribes to "fast-track" your appointment.

Ah, so, more or less I should make it retardo expensive for one of the low treatments and evenmore so(thronewise) or make it cost 2-3 Profit Margin's(unless you can suggest something better maybe please?)

If you want to put a price in thrones on it, I suppose you could just look at stuff like ... power armour as a guideline for something "basic" ...

Cytokinetic Therapy

A drug that slows down natural aging by a factor of 5, but which has to be taken weekly - with the aforementioned price covering use for, say, a year or so (basically, making you age only 2 1/2 months in that time).

Characters above the age of 100 would have to take the drug daily, with an obvious increase in cost (x7 to cover an entire year).

Characters above the age of 300 would have to take it hourly (x168).

Failure to renew the body's exposure to the drug in the aforementioned timeframe would cause the body to degrade rapidly, at a factor of

• x10 for characters below 100 years (aging 10 hours for every 1)
• x100 for characters between 100 and 300 years (aging a day every hour), and
• x10,000 for anyone older than 300 (aging a year every hour)

Which is why all the really old people in the Imperium run around with tubes stuck into their heads.

Then, a much more expensive and exclusive treatment beyond even most planetary governors could be ...

Regenerative Rejuvenation

Genetic treatment with progenitor cells that not only stops a character from aging but actually "dial back" time by slashing a character's current biological age in half. A rather painful procedure that will see the character hospitalised for several days or weeks, and is so invasive that it yields a number of Insanity Points as the subject feels their body reshape bit by bit and their brain is affected by chemical imbalances.

Includes complete blood chromatography as well as removal of excess/spent flesh to be replaced by newly formed muscles covered by vat-grown skin.

As an additional consequence, use of this treatment would double a character's aging speed, cumulative with future sessions.

Mind you, all this is pretty much off the top of my head (taking Space Marine longevity as a baseline for hypothetical details of rejuvenation), but maybe it serves as inspiration?