It depends. Promo singles are sometimes more expensive as they might contain exclusive tracks or they might be the only releases of that single. A lot of tracks have only been released as promos to radio stations, never for retail sale. The same thing with some promo albums. some albums have only been released as promo and then never officially released, like Dazzie Dee's where's my receipt, King Tees thy kingdom come, Homicide's album on Priority etc.
On the other side, promo versions of regular retail albums are less sought after since they often contain stamps, cuts, holes in barcodes or other features to prevent them from being sold as regular versions. A lot of collectors don't want any promos in their collections just because of that. So I wouldn't see them as more important than regular versions.
Of course, in these days and times of bootlegging and re-releases, the safest way of ensuring that you get a real version might be to actually buy a promo or a cutout of a rare album because then you'll know they are official