One important point to consider is to analyze how the database will be used.
LDAP is specifically tuned for searches rather than updates (add/modify/delete).
So if lot of updates (and transactions) are expected, then RDBMS is the way to go. But if search is primary (I remember statistics like, LDAP is very efficient if you do one or two updates per 100 searches), then
LDAP is the way to go.
Regarding APIs and ease of use and cost,
LDAP/mysql/postgresql, there is no big difference.
Hope that helps.
-k