I expect the real cheap flights are seats being dumped at short notice by charter operators. Since almost all their business is out-and-return loads, they usually have set durations (in weeks). You'd have to dig around some more for a month's stay. Might get cheaper the later in the season it gets if loads aren't what they planned for.
You're probably better looking in the travel ads in the paper or teletext than spending hours surfing the web.