Perhaps you need some kind of a special certification if you do facial tattoos --but back-of-neck-tattoos aren't that big of a deal.
I think it should be said that it isn't the PLACEMENT of the the tattoo that is what decides the cost of it --it is the size, and time it takes to do it, and sometimes coloring (depends on the shop you go too).
But tattoos aren't something to "shop around for the best price" on. Go to a shop that does good, quality work. If you pay $10 bucks for a a $50 tattoo, there is going to be a very big quality difference. --Most shops base their price in the hours. The shop my artist just moved to is like $50 base (or for half an hour), and then goes up gradually to like $400 for five hours.
Edit: Oh yeah, on facial tattoos --a lot of (A LOT!) of artists won't even do them because of all the nerve endings in your face, that if you hit them right, could blind you, paralyze you, etc.. A girl that used to work with my Mom wanted a little star by her eye, and she had to check out like four different shops before anyone would do it --she finally found someone who would, and didn't find out until -after- it was done why it was that no one would do it. ...She was kind of an idiot.