Sorry I've been away and missed this post, but Gaston has supplied you a lot of good answers.
http://www.cloudynights.com/item.php?item_id=2706
There was a solid write up about ONAG in cloudy nights; I think its a product I will buy into once (when) I become cashed up again. The price I gather for base set-up is around $1,000.
The only question I had was how easy is it to precisely focus the guide camera to the primary camera?
http://www.cloudynights.com/ubbthrea...Number/4951873
My challenge precision focusing the guide camera to the primary camera using an Lumicon OAG - required focusing the primary using a Bhatinov mask - primary using Rigel, guider using Jupiter! Only Jupiter was big and bright enough to let me see the diffraction pattern! Precision focusing the guider wasn't fun at all because the OAG didn't had a threaded tube mechanism to allow the guide camera to be adjusted by fractions of a mm at a time.
I think anyone designing ONAG or OAG should correct this by allowing a micro focus arrangement for at least one of the cameras attached to their rig! All it needed was a threaded sliding inner tube!
example
http://www.telescope-service.com/acc...l#schneckenfok