Longtails are key phrases that are usually 3 or more words. So, if you're targeting weight loss, without looking, I'll bet "weight loss", "diets", "dieting", etc. are all pretty expensive. However, "weight loss exercises and diet" would be a longtail, and is probably not going to cost you as much. It won't have as many searches either, but if you have 100 longtails, that will probably equal more searches than your primary targeted terms.
Here are some other longtails and their predicted daily searches from
Wordtracker:
healthy weight loss supplements 350
best weight loss products 332
south beach diet for free 300
best weight loss supplements 278
black cohash and weight loss 276
simply weight loss program 274
Those six add up to an estimated 1810 searches per day. They key phrase "weight loss" is an estimated 9120 per day, and "diet" is 2421 per day.
Overtures Bid Tool shows that the
lowest bid in the top ten bids for "weight loss" is $0.76
The same tool shows that the
number one bid for "healthy weight loss supplements" is $0.48.
So, there is a $0.38 difference that you can capitalize on.
Hope this helped.