A: How much more is it worth to you for a figure to be 100% complete?

It tends to be my requirement. I've seen the time and money spent by others trying to complete a loose incomplete figure and while there is some thrill in the chase, it doesn't seem worth it. (and sometimes more expensive)

I got a loose Blaster at a garage sale once, his weapon is fairly scarce due to it's fragility. I found it for sale by a parts seller at Botcon in 2011, he wanted $45 US for it. I didn't buy it.

B: Does it depend on the missing accessory? (example: you can live with G1 Twin Twist missing his gun, you cant live without G1 AM Soundwave missing Wingthing)

it does to a degree, I can't see Blurr being worth anything without the sheild, or a headmaster being worth much without the Headmaster. I eventually found a better condition Blaster with his gun at the parra fair for about $40. Sometimes it's worth the wait.

C: Does it depend if the accessory is necessary for full transformation? (examples: G1 Blurr's shield, G1 Erector's trailer)

Like a lot of others have said, if it's part of the figure in either mode for display I consider it necessary, if it's missing a spare missile that would be stored away otherwise then it's not such a big deal for me. even a G1 missile, if it's going to stand there with an empty launcher, that will bug me, I'd rather wait for a version of the toy that includes the missile so it doesn't bug me.

The trick is knowing if you can get hold of an accessory for a decent price, most often if a piece is missing it's something that's commonly missing or broken and getting a replacement part to fix or replace it could be the most expensive element of completing said vintage figure.

having said all that, I bought a few incomplete Pretenders last year as i'm totally aware that they are pretty crappy toys but I just wanted to check them out and see how the robots transformed and fit inside the shells. I dont' really have any interest in completing them, if I ever change my mind I'll probably look for complete ones instead of spare parts.