Thorg:
Neat, but confusing... Thorg wants to add silver inlay and perhaps a Bane (Undead) spell to his spear ... MAGIC makes it seem easy and inexpensive, but I know better. A silver coin or two should provide enough silver, inlaying it is the hard part... and then enchanting it with Bane.
So you want a Silver Inlay (Cheap, Minimal +2.5 CF or Cheap, Extensive +4 CF) ... or do you want Silver Plate (+2CF)? The difference is minimal cost, but vastly different book effects, one gives a Reaction Bonus when you sell it (Inlay), the other does "Silver damage" if you strike creatures that have a Weakness or Vulnerability to Silver (Silver Plate). Or, you can do both, take some form of Relief (Minimal +1.5 CF or Extensive +4 CF) and then Silver Plate it (total +6 CF, if you're looking to turn your spear into a Power or Endurance Item, which you probably aren't).
I"m pretty sure if you go for Cheap, Minimal Inlay on certain weapons, gwythaint treats it as Silver Plate (frex Chye's nunchuku). But I'm not sure that'll fly with a spear. Personally, say the head was Plated, the haft was Inlay, but that's me, I ain't the GM (I just really know the rules well).
So, for a regular spear (30$) you're looking at an outlay of +60 to 75 clams.
What's harder is the Bane. What enchantment is coming after the Bane? Because Bane is not an Enchantment that is used by itself. It's a Limiting Enchantment, it limits other Enchantments.
All "Bane (Undead)" says is "whatever other enchantments I have on this spear that were affected by the Bane, only work against Undead."
I presume you want Puissance+1 vs Undead? If so... here's how it would work
if gwythaint allows* it...
Bane itself costs 2,000$. It then reduces the cost of Puissance +1 from 5k$ to 1,667$.
So "Undead Slaying +1" on your spear will cost 3,667$ Note, if you ever want to make it stronger, Bane needs to be cast again along with the new Enchantments. So it's much cheaper (if you eventually want a bunch of enchants on it) to save up and go big all at once, so Bane only has to be cast the once. But even so, doing the enchantments piecemeal will still save a good chunk of change.
* Note, Bane is removed from
DFRPG as it makes it simpler on the GM, gwythaint however allows almost everything from base
GURPS,
DF, and
GURPS Magic (within limits). He allows the other books too, but those are teh ones that mostly impact magic Items (and of course
DFRPG Magic Items)