Tomislav, pornplanet.com would certainly be a nice domain (hence why the main extns are already registered) however pornpla.net doesn`t really work for me and for that reason, the value would be very low to non existent in my opinion.
I think it should be low $$$ imo.. since good xxx doamisna re already taken and thsi once kinda looks good.. That low $$$ is predicted only if you find teh right buyer who wants to develop this