In reply to andybike:
Anything that isn't mobile phone based is going to require some sort of sim card or subscription as it effectively will be a mobile phone that transmits your location using the mobile phone/data network.
The cost off set against a reasonable used smart phone may more than likely be the way forward.
A very quick search on eBay shows an iPhone 4 for £40 which is unlocked to any network. There will be some significantly cheaper than that...HTC etc.
Food for thought