I've never heard of getting 25-30% off the advertised price of any unit, just discounted from the MSRP on NEW units.
With used units it depends on mileage, maintenance, how old the tires are, and general condition of the unit.... carpets, paint, upholstery, etc.
NADA offers what I consider guidelines on used units but again, it depends on what other units are going for and their condition.
2011 Allegro 35QBA
Kar Kaddy KK460SS
2007.5 GMC, LMM D/A, Crew Cab, Z71 4x4, Long Box