Well I guess one of the reasons could be that the American Soul/funk/jazz records are indeed getting expensive because they’re sough after both by hiphopproducers digging for samples and hipsters/audiophiles who like having this music on vinyl for the sake of it.
Here in Amsterdam I do find a lot of cheap weird dutch or french jazz, obscure european film music, chansons etc, all of which often contain great sample material for cheap. Nobody seems to be collecting Reinbert de leeuw records for instance