you can only get the pro,pro advanced, and ultra plans with adsl2. you cannot get the $59.95 and lower plans with adsl2.
the cheapest adsl2 plan costs $79.95 and has a 15gb cap. but there is excess data charges of 2 cents per megabyte.
no xtra adsl2 plan has the option to purchase additional data blocks or have your speed lowered to dialup once your cap is reached.