Yeah -- it's a really bad setup in the end though because Groupon attracts the type of customer that isn't likely to spend a lot of money in the first place unless there is a deal. In other words, Groupon can attract a lot of customers, but the quality of those customers isn't very good (at least from my personal experience, all of the people I know who heavily use Groupon don't really use it at places they haven't already heard of before). The ROI must be terrible, but it also can be difficult to measure.
I think it depends on the kind of business. I think it is useful to break Groupon deals into a few categories, as well as the number of customers involved in any given deal:
FAST-FOOD DINING
Depending on the number of customers. most, if not all, fast-food restaurants will probably be able to make a profit of a 50% deal. These businesses thrive on large numbers of customers, and their profits scale extremely well. It is also relatively easy to win over new customers if you are more convenient, providing a reasonable opportunity to gain repeat customers (ex: Dominos does a deal and people find it delicious and cheap).
SIT-DOWN DINING
This is a much harder category to map a profit on a Groupon deal. These restaurants have a much more rigid number of customers they can serve on any given night. For a popular restaurant there is to use Groupon as they can already fill the seats and it is not a good value for them. For unpopular restaurants, people will attend but it seems unlikely these will be repeat customers. They are already bargain hunters, and it will be hard to wow these customers enough to return to your restaurant repeatedly.
ONE TIME USE ACTIVITIES
These deals have a similar problem to the sit-down restaurant. These are fun the first time, but repeat visits are much less compelling and it will be challenging to permanently pique the interest of bargain hunters. (ex: Deal on a boat tour)
REPEAT USE ACTIVITIES
Repeat activities seem like a reasonable set market for Groupon. These activities suffer from the same fixed-sized problem, however they stand a reasonable chance of retaining customers. Deals such as training and introductory courses are explicitly designed to attract new customers, and a Groupon top get them in the door seems fairly reasonable.
No matter what activity the Groupon is for, the number of customers who take advantage of the deal is a key component. Economy of scale is true for every business, and the more customers who take advantage of any given deal will directly affect the profit in involved with any given transaction
