A number of denominational presses are very small - they have very few titles available yet those titles are particularly interesting as they provide nearly everything available about the denomination. Others are small because most of the material about the denomination is not available in English. Often these materials are more large pamphlets rather than long studies so they aren't feasible on their own. But if you offered them as collections in the $30-40 range I suspect there would be a market.
You could test the market on the Eastern rites of the Catholic Church which are of interest to Catholics and Orthodox with additional current interest due to the Christian refugees flooding out of the Near East. Some publishers to consider:
