What is the cheapest wy to do it :)
Used!
Unfortunately, it still won't be cheap. You'll need the seat itself, and it'll have to be a 1500 seat (an 800 seat won't work), and you'll need the horseshoe bracket that supports it. You can see all of the parts here: http://fiche.ronayers.com/Index.cfm/Module/Main/TypeID/26/Type/Motorcycle/MakeID/3/Make/Kawasaki/YearID/45/Year/2004/ModelID/4101/Model/Vulcan_1500_Drifter/GroupID/128755/Group/Optional_Parts