Bob and Stu are both right.
With relatively new cloth on standard pockets, where balls can wobble and still fall and the cue ball can be drawn relatively easily, using a magic rack that turns 9 ball into 7 or 8 ball, then it plays around 640.
On a Diamond with broken in cloth and balls that haven't been polished for a few days, where balls will hang instead of dropping if hit a bit roughly, the cue ball must be stroked pure to get it to come back, and where they don't break open quite as nicely, then I'd say it plays around 700.
As Stu said, this simply means the level of player that would likely break even with it.