What's the appropriate amount to spend on a wedding gift?
$75 to $250 per person attending (range reflects how close you are to the couple). Less for destination weddings unless the couple are paying some of the travel expenses. For those who don't attend the wedding it can be per couple/family.
What's the least you can spend on a wedding gift?
If you're really financially strapped (and I mean really) you can spend as little as you have of course. But generally speaking, guests should stretch as far as their budget allows. I'd say $50 would be the minimum.
Does the amount change if you buy something from a registry, like if a couple has a $30 item on their registry, can a wedding guest buy that for the wedding?
No, the registry is not a dollar amount suggestion.
Do you need to give a gift of similar price to what the gift giver gave you?
That's not the point and gifts should be not payment but a something from the heart. That said, there probably will be an expectation of reciprocity if the couples are both in similar income brackets.
Why would a groomsman give a gift like two sets of cocktail napkins valued at $12 (see story here)?
Can you re-gift for a wedding gift?
If it's something special or unique, yes. Perhaps an antique or something you have that you know the couple covets. If it's just to cheap out, or something not particular to the couple that you wouldn't have picked out anyway -- no.
Why do some wedding guests give no gift at all?
I think there are dozens of reasons why people might give no gift. None are acceptable but some are understandable. For example: they procrastinated then felt embarrassed to send something late; they did send a gift but there was a delivery error of some sort; they didn't know what to get or how much to spend, or they were short on cash and so they put it off with the intention of sending something someday; they harbor secret resentment toward someone in the wedding party; or perhaps they're simply selfish.