Long tail PPC campaigns is a precarious issue IMHO. Long tail paid and organic search are not the same game.
The problem, in AdWords, is the minimum bid. You minimum bid is determined by your account history, Quality Score, and the history of the search vertical.
Why is it that a big company like eBay or Amazon can make such a killing off the long-tail? Because they have time. One thousand clicks that do nothing don't leave much of a dent in a campaign that generates tens of thousands of clicks per hour. But if you only generate one thousand clicks per month, the long-tail can be a death by tiny cuts.
If you have too many phrases that only generate a few clicks per week/month, their combined cost can overwhelm a small budget. The important thing to remember is that organic search and paid search have a major difference: organic visibility lasts as long as your website exists, but paid only lasts 'til you run out of money. If you are running a small to medium campaign you may find better results from running large groupings of negative words rather than from a long list of long-tail phrases.
A secondary concern while using the long-tail in paid search is that the more obscure your phrase, the less likely that you will achieve a high quality sore, unless you are targeting the phrase in your copy. If you have a low quality score, even an on-topic phrase may cost more than it is worth in the long run.
