Initiative isn't everything, and I wholly disagree with anyone that thinks the inish winner wins the fight. Bull-cakes. You're dealing with ignorant players if your experiences corroborate this claim, bar none, and no offense intended.
Frankly, I'd make it operate just like a Dex bonus item: bonus^2*1000 gp. Reason being: It's a straight bonus to initiative whereas a similar item to Dex would only increase the wearer's inish by 1/2 the item's bonus at best. (eg. a +10 Dex item increases the wearer's inish by +5 at best, right? Right.) while on the other hand it's only keyed to inish, not Reflex saves, AC, ranged attacks, et al. Once the bonus exceeds +6, it's an epic item and the price is multipled by the epic x10. Cut and dry.
Frankly, I'd make it operate just like a Dex bonus item: bonus^2*1000 gp. Reason being: It's a straight bonus to initiative whereas a similar item to Dex would only increase the wearer's inish by 1/2 the item's bonus at best. (eg. a +10 Dex item increases the wearer's inish by +5 at best, right? Right.) while on the other hand it's only keyed to inish, not Reflex saves, AC, ranged attacks, et al. Once the bonus exceeds +6, it's an epic item and the price is multipled by the epic x10. Cut and dry.