I always thought Suikoden was pretty niche compared to Tales, FF, DQ, Mother, etc... as jrpg series around that time. If you asked me how many copies a modern Suikoden game would sell today I'd guess like 150k worldwide?
Then again I guess if they made 5 of them + a tactics game + some weird ds thing it must've been selling alright to keep it funded. Though by IV it felt like most of the funding had dropped out.