It's certainly possible to replace the bearing, but unless you're doing the work yourself (and have the proper tools to press the old bearing out and the new one in), I suspect it would be actually cheaper to buy and install a replacement fan.
Figure $10-$15 for the new sealed bearing and a minimum of 2 hrs. shop time (esp. if you have them do the removal/installation work) @ $65/hr. (minimum) and you're right up there with replacement cost anyway, but without the new part 2 yr. warranty from Porsche.
