You can also get a do it your self System from well known amateur astronomer and telescope maker Mel Bartel.
He sell 2 systems a Servo and a Stepper system .
All you have to do are some modifications to your existing Dob base .
It is most probably a better alternative if you have some scope experience but plenty of newbies have performed the changes required.
The cost would depend on how much you can source your self or he can supply all the drive components for you to fit and modify to suit your Dob.
The systems work stand alone or with a computer for full Go-To. You supply the computer.-, and it can interface with popular star charts.
You can have push and track or full Go-To and alt Az or EQ track.
It is a very versatile system that does every things required and is intended for the financially challenged of us or if you enjoy doing it your self.
I have included a couple of links.
I have one of the Servo Systems and am in the middle of building a rather large fork mount for my Newtonian.
Pics will be posted as I near completion.
http://www.bbastrodesigns.com/#limitedwarranty
http://groups.yahoo.com/group/scope-drive/
There are literally thousands of his systems around the world and some of the commercial ones available are variants under license of his Systems.
The other alternative is to buy one of the commercial systems mentioned by Iceman .
Some of these come in a kit to suit a particular Dob manufacturer and don't require as much self input.
Even if your not interested in do it your self ,there are lots of info in the links to explan Go-To Dobs and scopes in general.
Mark