I am going to go against the grain and say I think it is a great gesture to buy the boss a gift as a group.
I can see how it might be awkward if you set a pattern and then it doesn't happen though...
I guess I see it that I know several people in my workplace are going to buy a gift for the boss so why not all chip it together and get something of more value?
We have always then all signed the card even if one person only can afford to put in $1 and another can put in $30
I think it is a way of saying thanks for putting up with us, giving us opportunities, assisting us, training us, etc etc.
But maybe that is industry dependent.