What would be an appropriate amount for a DM to charge a PC to upgrade a magical whip into a mighty whip dagger (Arms and Equipment Guide p 10), maintaining the same properties as the original weapon, but not adding any additional magical properties?
The whip is currently "Flay" from Weapons of Legacy. A +1 whip worth 4,300gp that has some additional magical abilities.
Mechanically, the difference is damage. A regular whip does 1d3 subdual damage. A mighty whip dagger does 1d6 + STR_MOD damage.
Flavor-wise, the difference is the whip has a dagger-like blade at the end of it.
The player would like to have it turned into a whip-dagger so he can use it more often in combat. The upgrade is almost entirely for flavor, since he could just as well shoot his short bow when he's 15 feet away and wants to do damage.
The upgrade may save the PC a move action from time to time, but the main benefit is flavor and style.