Yeah, it all has to do with what kind of relationship the person has with the vet clinic, and how much leeway the clinic has on certain things. If the Rabies Lab will take owner requests directly, that's great, and will probably turn out to be the least pricey way to get a rabies titer done.
The subject of costs can be a touchy one for client and clinic alike. I ended up taking Charlie Bear to a rescue vet this summer for his bladder-stone surgery, because the estimate from my vet clinic was one I felt was unnecessarily high -- and because the rescue really should have dealt with the stones before it adopted CB out. (However, the rescue was in a turmoil over the sudden death of its director, so I'm not blaming, just noting.)
My clinic's prices are on the higher end, and I know it. The tradeoff is that they never, ever argue with me or lecture me on the way I do things; they are willing to let me forego things like rabies vax in favor of titers; they always back me up on things; and this weekend when Beau laid his leg open to the tendon, they said 'bring him in now' when I called, they rearranged the doc's schedule for the next few hours so he could get Beau into surgery, and they spent A LOT of time and care on him at a time when I was completely freaked out. (The surgery was $500, BTW, which I thought was a pretty reasonable cost.) Add to that: my vet calls me to follow up after every visit that is not routine -- to see how Pip is doing, or how Charlie's eye is, etc. We know each other, I inquire after his dogs and his wife, we are both White Sox fans, we have a nice business-friendship going.
I know this is OT, I just am thinking out loud about how the costs are only one element in a relationship, and trusting skilled people to look after my dogs does create a relationship. I would love to pay less than $100 for a rabies titer test, but I am not able to do my own blood draws, etc., so that will probably never be an option for me, nor will I break down my clinic's services into each component. Because I don't want them to say, when I am looking at my dog's flexor tendon and gagging, 'we can get him in next Tuesday at 3 p.m.' -- and yes, I do know clinics that would do that!!