Is that just for doing blood and sending it off to KS? I paid $255 (~$125 each) to get my dog and cat done (not including microchip/vaccine), so yes, $300 for 1 pet seems high.
KS charges, I believe, $47 to perform the test, so once you factor in the vet visit + shipping to KS, ~$100 per pet is pretty reasonable. $300 per pet isn't quite as reasonable, though.
It also may depend on regional pricing, too, since I paid $134 total for 1 microchip and 2 rabies shots to get my dog/cat prepped for the bloodwork. If your vet would charge more for your dog to get those 2 procedures done, then they may just charge more in your area.
Hope that helps!