In general, with frequencies as high as those used for 802.11b or g wireless networking, the coaxial cable is extremely expensive, and often hard to get. Since the connectors you need are so tiny, and with such fine tolerances, they're a bear to assemble properly. Finally, a 5m cable is pretty long for the 2.4GHz range, so it's probably very fancy cable-with a price tag to go along with it.
My bottom line would be to make do with a much shorter run (2m is about as far as you can go without lots of loss), or bite the bullet and shell out the $50 for the cable you want.