You have to offer a minimum of 50% discount, although they'd prefer if you offer 60%+, I've heard that they'll lead you to inflate your prices before discounting them (so your not really offering such a massive discount). Then they take 50% of whatever the customer pays, so if customer pays £22 for 2 rooms, you clean 2 rooms for the grand old sum of £11 (minus tax of course..) I've taken part in a few groupon deals, with very little intention of spending anymore with the retailer, so I think the chances of add-on sales are slim. You may make some money on people taking the deal and not using the voucher by the expiry date (I've also done this - doh!). Basically, tread with caution!
Ash