You're all looking at this the wrong way. Forget about discounts and what you normally charge.
How long would it take you to clean 2 rooms ? An hour ?
So that's £17 for an hours work - not a fortune granted - but more importantly it's cost you nothing to get that client and from Jason's point of view a new client costs £40 to obtain.
Plus the money is guaranteed.
If you are any good at selling, then once you are in the clients home you can discuss additional work, maybe to be done at a later date. Maybe you could tempt them with a further discounted deal for future work, obviously not as much as the original deal. If these are the type of clients that want a deal then give them what they want. It doesn't automatically follow that because they want a deal, they will have minging carpets.
Plus it doesn't matter how good the offer is, the Groupon client is only going to book it if they happen to need their carpets cleaned at that particular time.
The biggest risk with this type of deal is not being able to cope with a massive influx of work to be done within a limited time frame.