I once read article about this, and yes, some power supplies work when you connect something between 12 and 5 wolts. But the writer of that article had got mail from some PSU manuf. telling him that some PSUs could even get damaged. Older PSUs are more likely to work with this hack. And also more likely to burn instead of shut down
Oh, and connecting ground to 5v wont work. Your PSU will shut down or burn.
Resistor would be your cheapest way. Size depends on your fan amps.
Calculate like this <desired voltage> / <fan amps> = ohm
So to get 7V for 0.2A fan you get 7 / 0.2 = 35ohm resistor. Also at 7V you are cutting 40% of the wattage from the fan to the resistor so with 10W fan youd need 4W resistor. Their likely to sell you .25W resistors if you dont ask specificly. You can calculate the fan wattage by <fan amps> * 12V. So you likely dont have a 10W fan
More advanced would be to connect a large (>=4700uF) condensator (or is it conductor in english? Well just say "something with 4700uF" where uF is "microfarade"

) in parallel with the resistor to make sure that the fan gets 12V at startup to get spinning. Specially if you use low voltage.