On giving gifts to LOR writers
Here's how I would feel about various expressions of thanks, for something like writing a letter of recommendation:


No response at all: I might be a little disappointed but if I am busy I might not even notice.  Some people might be more offended.
Email saying thank you: Feels perfectly sufficient to me.
Handwritten note: A very thoughtful gesture.
Token gift (small box of candy, desk trinket, etc): Appreciative but slightly embarrassed.
Gift of non-negligible monetary value: Quite embarrassed.  I would probably gracefully try to decline it.
Gift of money, in any amount: Very uncomfortable.  Feels like a bribe.  I'd refuse.


Of course this could vary by culture (I'm in the US).

Regarding gifts for an advisor: A common tradition is to give your advisor a nicely bound copy of your dissertation, for their reference and as a memento.    (Also be sure to thank them in the dissertation!)  You could accompany it with a nice personal note.  But again, I wouldn't suggest anything of significant value.