re: Typical labor cost to install 3x6 Subway tiles

Backsplash shouldn't cost much at all. Backsplashes aren't considered wet areas and the tile can be adhered directly to the drywall with little or no prep.

A shower is obviously a wet area and prices vary widly depending on who is doing the install. Kerdi showers done correctly can easly run $10k. I've done a Kerdi shower and it was a PITA and can understand the extra money tile contractors charge. For my own shower I used the laticrete hydro ban for a big savings. The key to doing a shower install is the waterproofing. The cheapest route I would go is redguard from Home Depot. Kerdi is the industry leader but unless your building a steam shower I feel redguard or laticrete will suffice.

Cost for put will depend on what you choose as your waterproofing and if you are paying for it or the installer is paying for it.