The price does seem high, based on having paid for 3 liner installs last year, but I don't know the particulars of your install. One will often under-size the pipe if the chimney is very tall. Draft at the stove is a combination of height and pipe cross-section, and in general, the more you have of each, the stronger your draft will be. The hazards of going too large in pipe diameter are insufficient air velocity in the pipe (cooling and creosote build-up), and too much draft for your stove (over-firing). If your chimney is the usual 16 - 25 feet, you'd better stick with an 8" pipe for an 8" stove. If your chimney height is over 30 feet, then reducing to 7" may be warranted, but I'd take that advice directly from someone who has experience with a BK King.