I would consider £129 if that included a subscription to the service.
I use a tracking device on my motorcycle. That cost about £300, and costs me £90 a year for the subscription. However, for that I get texts and emails if it moves without the keys in the ignition, they'll liaise with police if it gets stolen, and it also logs my journeys so I can log in and review where I've been - those logs don't expire unless I let the subscription lapse.
Simple tracking devices are available on
ebay for about £50. You send it a text, and it returns your GPS co-ordinates, but that's it.
Like others, I would expect it to be hidable - Ie, live in the seat tube or something. You'd also need some way of defining whether or not the bike was legitimately on the move or not. Easy for a motorbike - is the ignition switched on or not. I'm not sure how you'd do the same thing on a bicycle. Charging point would have to be discreet so as not to make anyone aware that it was there. Perhaps you could use induction charging?