A: How much more is it worth to you for a figure to be 100% complete?
as a rule , i won't buy anything unless its 100% complete just my personal preference, unless the incomplete figure is dirt cheap or something i can pick up locally at a bargain

B: Does it depend on the missing accessory?
sometimes but only in the case that its an extra missile for a gun that will be filled already etc.
C: Does it depend if the accessory is necessary for full transformation? (examples: G1 Blurr's shield, G1 Erector's trailer)
definately , i have a g1 wheelie thats missing his 'cover' and it shits me that it isn't there