Knowledge is power. Do as much research as you can on prices. Have a look at online dealers like Coast 2 Coast. (Never used them but they've been around a while.) Have a look at second hand prices.
Have a look at this.
http://www.moneysavingexpert.com/car...buying-new-car
(I know I always link from the site but there's a lot of sense written.)
PS Let us know the make and model and I bet someone here will find a cheaper one for you!