Why not do a bit of work yourself?
It's easy to determine what a ticket using LM would cost you. You can easily figure out what you are paying for the miles and how many of them it would take you to get to a specific destination (the prices in miles are a fixed amount based on country). Compare that to a ticket you buy and you know what the difference is....
Just don't forget by buying miles you are dependent on award availability within Star Alliance, some routes have very limited availability. Cash will get you on any flight that is available and you will also earn some miles.
So if you don't know the rules, first do some reading. Then be flexible and you might get a good deal. But do know what you are getting into when buying miles. These programs do have a lot of restrictions.
For a newbie I would suggest you also look at buying US Airways miles... That program is a bit easier to understand.