IMO, £6 per hour is not reasonable, but then it is your money and you can and should do with it what you wish.
To the more objective side of things, power measurement is often sold as some sort of magic metric that is used like an expensive speedo but all of a sudden allows you to train more effectively. This is not the case, the power readout will not offer much, if any benefit over a turbo session riding to speed (if your turbo has a repeatable speed vs power curve, you can get an intuitive feel for if this is the case by riding it regularly) or heart rate. It might actually be a thorn in the side if my experience of Watt bikes is anything to go by.
Do some reality maths, how many times per week will you go? How much does it cost per session? For how long will you keep it up? Are there any other extra costs such as travelling to the gym etc? Add it all up.
How much does a powertap wheel cost? (PT chosen for example as it is a cheap power meter, you can look at other options)
If the total cost =>50% of the powertap wheel, buy a powertap use it for as long as you want, then sell it for 50% of what you paid. You will be financially even to the wattbike scenario at worst, yet you will have been able to use it for every ride and use it like a power meter ought to be used (not one off sessions).
FWIW I use a power meter for every ride and am massively pro training with power, but having now been training and racing with power for as long as I have, I know that it doesn't work like magazines etc would have you believe! I've also got a fair bit of experience using a watt bike.