Kadees are 4 bucks for 2 pairs retail with most shops discounting them for about $3.50-$3.75 plus tax. They're not exactly bank breakers. I usually keep at least one pair of medium overset shank, medium underset shank, long centerset shank, #148 whisker couplers and #119 shelf couplers.
The overset and underset are great for some equipment where it's not easy to modify the ride height, such as with locomotives.
The long centerset shank couplers are great for locomotive pilots. Some brands such as Athearn, Walthers, and Intermountain make locomotives where the plow would get in the way of the trip pins on the couplers causing uncouplings on curves, or cars would refuse to couple to the engine.
The #148 whisker couplers are for cars where the coupler pocket is too shallow to accept a regular bronze centering spring with a #5.
The #119 shelf couplers are for tank cars.
If I need anything else, such as a long underset shank coupler, I'd usually go to the LHS for those, but Kadee hasn't been making them for a while (they're on the list, but is always unavailable/sold out when the LHS tries to order them) so I have a small stash of them that I received from forum members a while back.