I just bought a set of platinum plugs for our '09 V10 for under $12 per each. It's not the plugs that are costly, it's the labor to re & re them that is.
Each plug has it's own coil that needs to be removed and the boot on each should be replaced. The earlier V10s had insufficient number of threads in the plug hole (hence some plugs blown out) and if you don't do it right, you can seriously damage the head or end up with broken plugs that need a special tool to remove them. The recommended procedure is to put some penetrating oil into the holes (Kroil, WD40, etc.) and wait 20 minutes and turn the plug 1/8 turn, put more penetrating oil in and wait another 20 minutes, turn another 1/8 turn and put more penetrating oil in and repeat as many times as it takes to get the plugs out (times and turns might be different as I'm going by memory). Supposedly takes a Ford tech. 3-4 hours just to remove them. The newer V10s are much easier as the heads got redesigned.
So that's why the spark plug bill is so high and I don't think it's out of line. But do you even need a plug change yet? You might try the
Ford Truck Enthusiasts Forum.
What is included in the axle and transmission service? Just a fluid change or new seals? Ford is probably about the most expensive place you could go. I would get at least a couple more quotes from regular shops.