prices vary.
locally $250 - $275 per duct is the cost here.
the rest could be lots of things including that
it is a real pita to change ducts in a crawlspace.
hard to work lying on your side or back, crawling
around in the dirt.

some companies won't send worker to make $550
for two (?) hours work when they can make more
for the same amount of time installing bigger ticket
items.

get another bid, or diy.

and know if one duct has fallen off/apart/whatever,
that the rest probably could use some work.

best of luck