Mate, the price garages are going, thats not too bad. I know its a simple rubber stopper, but they way they look at it is they charge say £40 an hour. It took half hour to remove the wheel, tyre, put the stopper in, put the tyre back onto the rim, balance, and back onto the car. They prob say half hour labour and £4 for the rubber bung.
I always get work done like that buy a mate. I know a few lads that will do it for me. I was shocked when I had my cat put on before my MOT. That cost £35!!!! 4 bolts! That is £8.74 per bolt to take out and put back in!!!
Garages are a rubbish now days. I remember that anything simple like that they would do for next to nothing, so they would get your custom the next time.