Hi, in Italy we still have some of these (lovely) cars, so are not so rare;
moreover, for a recent law here cars are vintage/classic only when they have
at least 30 years (before it was 20 years, with reduction of taxation and insurance),
so actually a 33 16v P4 is assessed quite low at about 1500-3000e, also because
of complex maintenance and high taxation; here one example at 2000e:
Alfa romeo 33 - 1993 1.7 16v q.v Auto usata - In vendita Ascoli Piceno
Probably in Italy prices will grow a lot when the model will have 30 years.
There are also some few professional dealers that buy a 33 that
was on internet and it was already a difficult sell at low price
and after they put the same car on internet at extremely high price (x2 or x4),
99% never sell the car at that price, probably they just do a try.
But a better part for you is that out of Italy I imagine it is very different,
the P4 can be quite rare and exotic out of italy, so it will be assessed at more ( x2 ? )