The autostun effect of the Horn of Blasting is quite comparable to 8th and 9th level combat spells. It is certainly better than the 7th level Power Word Stun.
As such, the bare minimum value of the HoB is 100,000+ gp.
I think the original concept of the HoB could work in a campaign where the players never learn of its limitations until it was too late -- it would not be long before it blew up in someone's face. Amusing to the DM that, and quite fair as well.