Well, let's see. What is involved?
The sample, the lysis buffer (cheap), a pile of various plastics (semi cheap), protinase K (kind of "cheap"), a heat block to keep at 55c (5K), some ethanol to precipitate the DNA (cheap), centrifuge that can do 10k+ RPM, some clean ddH20 (15k for a new system), primers for the sex chromosome (really cheap), a pcr machine (5k), a gel running set up (a few hundred). Averaged over time, over head and man hours will be the expensive part, so $20bucks a sample doesn't sound unfair to me. Your small french fries cost 1.29, but you could buy a 10lb sack of potatoes on sale for the same price.
Guess if the company was really high tech, they can use illumina genotyping, but that's $$$ for the facilities.
Edit: Just took a look at avian biotech, looks like they use regular ol pcr to do their testing. Safe, simple, tried and true tested method used for decades. $20-25 is a fair price for the job. Running a lab isn't cheap. I'm actually waiting for a pcr at the moment to finish the batch of genotyping on 92 samples.