I just shopped around for one this summer - was $375-$650 depending on where you looked. Remember that if you go through a third party - the price they quote you is for the data connection only. The loop still has to be paid for as well so you may as well just go ma bell all the way unless your third party provider gives you something in addition that the phone company does not.
In our case it was cheaper to increase the number of DSL lines from 2 to 4. We aren't mission critical so no biggie. Mostly do backups, ftp, vpn, etc through the other 3 that are not used for general browsing.