I've always been interested in this question too, and I think the honest answer is that it depends on what Kirby signed. There are Captain America prints from 1976 that Kirby signed that don't go for that much money because there are a lot of those signed prints out there. Kirby in the later years worked on an Independent book, whose name I'm now blanking on, but when you look at the artwork, you see Kirby and his influence, and there are Kirby signed copies of those independent comic books floating around that don't sell for *that* much given that it's a Kirby signature.
Years and years ago, Wizard had a failed experiment where they tried to assign a value to everyone's signature, and Kirby's was among the top at that time, at $500, I think. This "signature price guide" lasted a total of 2 issues, if I remember correctly, and was quickly pulled and never discussed again.
I would think, for a Silver Age Marvel book, it could add hundreds (or even thousands, to the right book) of dollars in value, but it's so subjective. You find that one Kirby collector that's gotta have that signed comic, and you can set your price.
To answer your question, I think the Avengers #3 and FF #5 would see a substantial price increase from the signature (definitely get Stan to sign the FF #5) if you can wait for the right buyer. Throwing it up on eBay, probably at least a $200 to $300 bump, maybe more. Just my guess.
I only have 1 Kirby signed comic, FF #4, probably in G or G+, I got Stan Lee to sign the splash page along with Kirby, it's one of the 3 books I doubt I'd ever sell for any amount of money. Hmm, I never thought of getting Stan to sign it again and CGC SS it. Food for thought...
I'm interested to hear other people's thoughts on this topic, I think I've rambled enough.
Joel