Cost to upgrade an enchanted whip to a mighty whip-dagger?

Question

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.

Answer 1

The Dungeons and Dragons 3.5 revision allows a whip's wielder—and, by extension, a whip-dagger's wielder—to deal damage with the weapon using the wielder's entire Strength bonus. Both whips are pretty much just melee weapons with 15-ft. reaches that provoke attacks of opportunity when used. (Prior to the 3.5 revision, the whip was an even weirder projectile weapon hence it normally dealing no increased damage because of the wielder's Strength and needing to be made mighty like a 3.5 composite bow for a wielder to use his Strength bonus with it.) In other words, the 3.5 revision makes a mighty whip-dagger—like a mighty whip—unnecessary: a 3.5 non-mighty whip or non-mighty whip-dagger does exactly the same thing a Third Edition mighty whip or mighty whip-dagger did.

That said, changing one weapon into another—even a change as seemingly simple as tying a dagger to the tip of a bullwhip—is something best discussed with the DM. To do this right will likely require the help of a high-powered wizard who casts dispel magic on Flay to make it nonmagical for a few rounds and follows that by casting polymorph any object on Flay to transform it from whip to whip-dagger. There's no more provision for this than there is for turning a mace into a longbow. (But also see this question.)

Note that the whip-dagger was printed twice for Dungeons and Dragons, Third Edition in Sword and Fist (71, 74-5) and Arms and Equipment Guide (6, 10) and thrice for 3.5 in Dragon Compendium Volume 1 (126), Dungeon #134 (50), and, finally, in Dragon #353 (28). This last is the one you should use if you're trying to be up-to-date on everything.