Cheapest way is obviously just using the built in popup flash as a CLS commander, usually the easiest way too, but it does have issues (line of sight, it still shows up as a reflection in glass/metal/etc - but the SG-3IR sorts the reflections at least).
I recently decided to pick up a TTL cable (I got a lastolite one cheap, works a treat), and it's great. Lets me now have up to four groups of flashes in CLS (of course, one of those groups is a group of 1 flash, which is the commander).
But, I'd go the CLS route while you're getting started, and you're not forced to use iTTL, you can still set the off-camera flash using manual power settings from within your D700's menus.