For a decent TPMS sensor (Huf, Schräder for example, these are brands also used by a lot of car manufacturers), programmed to your car, around 60€ per piece would be a fair price, imo, if done by someone who knows what he's doing.

At least, that's the price we charge for a TPMS sensor ;-)
You can always go cheaper, but then you might face quality (corrosion) or lifespan issues (battery) (mind the 'might', not 'will').
Also, ebay doesn't mount them on your car... Unless you have the equipment to mount tyres on rims and balance them properly, you will have to go to a tyre center or so with your internet-bought sensors. They will not mount them for free either...
Whatever you buy, quality and service have their price.