Its probably a good idea to consider the increased price of the Mithril Weapon basically offsets against the cost of the bonus ATK.
Mithril Short Sword 250 Mhl / Short Sword 40 Mhl
Mithril Rapier 300 Mhl / Rapier 40 Mhl
Mithril Long Sword 350 Mhl / Long Sword 60 Mhl
Mithril Scimitar 350 Mhl / Scimitar 60 Mhl
Attack Enchantments cost 100 Mhl for each +1.
Normal weapons max at +5 ATK, costing 500 Mhl altogether.
Mithril Weapons max at +4 ATK, costing 400 Mhl less 25% Mithril discount, which is 300 Mhl.
Hence the following would be the full cost for each pair of fully enchanted items at +5.
7ATK : Mithril Short Sword 550 Mhl / Short Sword 540 Mhl
7ATK : Mithril Rapier 600 Mhl / Rapier 540 Mhl
8ATK : Mithril Long Sword 650 Mhl / Long Sword 560 Mhl
8ATK : Mithril Scimitar 650 Mhl / Scimitar 560 Mhl
As you can see, the cost and benefit doesn't weigh out. You pay more at the beginning, and even fully enchanted, you STILL pay more. So why buy a Mithril weapon ?
Well, Mithril can hold up to 3 enchantments (from
http://belariath.com/shops/ss/enchant.html ) and you have savings for ALL THREE of the Enchantments.
In other words, you can put +X ATK / (some Enchantment) / (some Enchantment) on a Mithril weapon / armor, and
still have the 25% discount on those Enchantments. This adds up to a lot of Mhl saved if the Enchantments are actually Spells / Enchantments with multiple slots, and you save 25% on each and every slot.
Spell Enchantments: 50 Mhl for the first slot, 100 Mhl for slots 2-5, 250 mhl for slots 6-10, 750 Mhl for slots 11-15
As you can see, you will be saving about 62.5 Mhl per slot from Slots 6-10, and 187.5 Mhl per Slot from Slots 11-15. Spellcasters can also save a large amount of Mhl on Focus Enchantments using a single Mithril item as the "container"
Information in Quotes is outdated as per information by Dev
Its important to note that (unless rules have changed since), New Enchantments may not be added to an already enchanted item, without first disenchanting it completely, although Existing Enchantments may be strengthened.
In other words, a Mithril Sword +1 with enchanted with +4 ATK will need to be disenchanted and re-enchanted to become a Mithril Sword +1 enchanted with +4 ATK / Firebolt 15, ie you are paying a second time for the +4 ATK. However, a Mithril Sword +1 enchanted with +2 ATK / Firebolt 3 can be upgraded to +4 ATK / Firebolt 15 simply by "topping up" the cost of the spell slots onto the item.
"But I don't have so much gold!"
Yes, so its best to plan ahead what you want to put on a Mithril item, and start by enchanting it with 3 Enchantments, but with just 1-2 slots. From there, as you get more Mhl, you make the slots stronger
Hope that helps explain the more practical aspect, the unspoken question of "How Does Mithril Weapon / Armor save me Mhl?"
In short though : Don't Buy Mithril unless you have Enchantments you'd like to Get on it in Future.
Prev Stand prior notice of Enchantment Rule Change wrote:In short though : Don't Buy Mithril unless you have a Lot of Money and Enchantments you'd like to Get.