Shuttles are cheap, but take FOREVER! Both times I've taken shuttles to the strip, I've waited over an hour for them to depart. Not fun. Especially in the summer time.
Cabs are usually pretty cheap. Last month I took a cab from LAS to the MGM and it cost $8. And it was instant. 2 hours later, JordanD took a cab from LAS to the MGM and spent about $30 because of "traffic". You won't always get an honest driver, if it seems he's giving you the run around(its almost a direct route from the strip to the airport), say something.
And if you do take a shuttle, keep in mind they aren't 24/7. I bought a return ticket on the shuttle at last years NJC, but I was flying back to SFO on the first flight of the morning. It was just awesome when I call to inquire about the shuttle and I'm told I'm going to miss my flight because they aren't running yet. Took a cab, $10.