Digital Boxed Sets and Omnibus Editions – Are They Worth It?

I've taken the Eye of the Storm Omnibus Edition offline with the intention of updating it, however, I checked my stats for the edition and ... almost no one buys it. Over the past year (and 3 years) there have been maybe two or three sales of it per year, if that, even at the lower price of £9.99. Most people tend to go for the individual series volumes instead – then you can buy at leisure every month when you have the money, I guess ... or you can decide to stop the series if you reach halfway...