The chip itself is cheap, sub-$30 ($19 sounds about right, but it may be a little higher). You do need a vet to do the injection, its a gigantic needle that needs to be placed just right in order to get the chip embedded in the correct ligament. You also should have a chip reader to verify that the chip is working and the correct ID is displayed. You can buy the chip directly from the manufacturer, then have the vet inject if for you. Most vets won't/can't buy them for you, from what I've heard (why that is I have no idea).
Don't forget to figure out what chip to buy. The Equine Working Group (the industry arm of the equine version of NAIS) disagrees with the frequency to be used. ALL US horses at this point have been microchipped (if they have one) with a 125mhz chip. Small animals, livestock, and imported horses (done overseas) are chipped with 133mhz chips. The two are not compatible with each other, although there are readers that can do both. The current industry standard is 125mhz, the EWG is recommending 133mhz. Which will it be?
And don't forget to protest NAIS if you disagree with the idea of having your horse tracked every time you leave your propery. I'm all for the idea of nationally mandated microchipping (look how benificial it was to LA during Katrina) but the idea of logging my horse in and out of the farm seems a huge invasion of privacy to me. Educate yourself closely and look at both sides to see through the propaganda BOTH sides are spewing. We don't have to accept NAIS if we don't want it, but in order to do that people need to make themselves heard NOW. Its the will of the people, and the more people learn about it the more people are shocked at the invasion of privacy that is being put into law without people even knowing about it. I'm for and against it, so I'm trying to present a middle opinion, but try your best to get unbiased information, there isn't much of it out there in regards to NAIS.