$US 5250 sounds ballpark right for a decent brand new spinnaker
for a 43 footer...
But, as has been mentioned before, buying second hand is a much cheaper option.
Personally, I'd try and find the Farr 40 OD owners in your area and see if any of them have cheap
spinnakers for sale
Farr 40 OD was (and kinda still is) a very strong one-design keelboat class. Three or four years ago, they changed the class rules to allow masthead spinnakers, and overnight, all their fractional spinnakers became essentially obsolete. So lots of opportunity to buy Farr 40 OD fractional kites for a fraction of a new kite price... we have 2 of these puppies (0.6oz all purpose and 0.5oz runner) that we paid $500 each for... and they were in pretty much "as new" condition.
In general, people who can afford to campaign a Farr 40 OD can afford to update their sails
regularly and don't really need to squeeze every dollar out of on-selling older sails