I'm in Canada and I pay around $80-100 for just a vet visit without extras, so $100 in the US sounds like it might be pretty steep.
I wouldn't give the rest of the doses to Breckenridge, and maybe go to another vet who's more knowledgeable on hedgehogs. Ivermectin isn't the only treatment for mites, but it's known to be a deadly one. It's not worth risking the health/life of an animal when there's other viable options. Revolution is safe for them and will still get rid of the mites.
Here's a link to a previous thread
with more information, specifically one that only gave the first dose of Ivermectin.