Hmmmm? The buntline barrel length will depend on which model it is - whether a Frontier Scout, Peacemaker or New Frontier. By model the buntline 22 rimfires were: Scout 9 1/2", Peacemaker 7 1/2" and New Frontier 7 1/2". So if it's got a 12" barrel, it's a custom barrel added by someone.
I had both Scouts and Peacemakers with shorter barrels, but the New Frontier convertable I bought for tree rat hunting soon after they came out (about 1970-71) was a 7 1/2" Buntline.
What's it worth will be dependant on condition, which model, vintage and how bad a buyer wants one. So $400 could be either way high or low depending on those factors.
FWIW, I had two sets of custom grips made for my NF by David Wallan and when I sold it in 1996 I got $650 for it in about 95% condition with all three grip sets.