This is covered in the end-notes for the novel Dune. Melange could cost as much as 620,000 solaris for ten grams although, presumably it was usually much much less expensive. Typical dosages for an addict (like Piter) could be as much as 2-3 grams per day.
Given that a Solari appears to be roughly as valuable as a US dollar (a few will buy a small doll or rent a taxi, a few hundred will engage the services of a prostitute for an hour) it can be assumed that a wealthy individual with an income of a million-plus solaris a year might be able to afford a decagram of spice annually, using less than one gram per month (or possibly giving themselves a few larger doses every quarter).
The average common citizen would never be able to afford even a single full dose, having to settle for highly adulterated spice coffee or nothing at all.
MELANGE: the “spice of spices,” the crop for which Arrakis is the
unique source. The spice, chiefly noted for its geriatric qualities,
is mildly addictive when taken in small quantities, severely addictive
when imbibed in quantities above two grams daily per seventy kilos of
body weight. (See Ibad, Water of Life, and Pre-spice Mass.) Muad’Dib
claimed the spice as a key to his prophetic powers. Guild navigators
make similar claims. Its price on the Imperial market has ranged as
high as 620,000 solaris the decagram.