Interesting idea. I seem to recall some spells in Magic of Faerun that magically coat skeletal bones in stone or iron...they might be good places to start. Sadly, I forget the names/page references.
Failing that, I'd suppose a decent rule of thumb would be something like:
Bronze coating - Reduce base speed of skeleton by 2/3rds (as if wearing heavy armor). Add +4 armor bonus. Give +1 to hit and damage with natural "claw" attack (on the assumption that metal bones are tougher and sharper than natural ones). Costs, say...100gp per hit dice of skeleton in bronze.
Steel coating - Reduce base speed by 2/3rds. Add +6 armor bonus. +1 to hit and damage. Costs 150gp/hit dice.
-- The reasoning here that steel wouldn't really do more damage than bronze as a weapon (though it would most definitely break more easily and dull faster).
Mithril coating - No reduction to base speed. +6 armor bonus to AC. +2 to hit and damage. Costs 500gp/hit dice.
-- While mithril isn't any stronger than steel, it IS much lighter. This allows thicker, sharper "blades" to be formed over the skeleton's claws without hampering their movement.
Adamantite coating - Reduce base speed by 2/3rds. +8 armor bonus to AC. +2 to hit and damage. 750gp/hit dice. Also, since adamantite is generally considered very rare, outfitting a lot of skeletons would probably cost a lot more than 750/hit dice simply because you quickly start exceeding the ready supply.
Note too that the costs listed should only include the raw costs for the materials. There may be magic costs as well (especially since the melting points of steel and presumably mithril and admantite are very high) to shield the skeletons from the heat of immersion...or, alternatively, to magically sculpt the metal around them, perhaps using a variant of Stone Shape or a completely original spell/item.
