I know Facebook, MySpace and Twitter are great ways to help promote my company, but I don't have the time to learn them. How much would a marketing firm charge me to run my social media?

You can find a firm to handle your social media for as little as $500 a month. However, what you might want to do is create an E-Marketing Plan. This would distill your company's brand, set forth your objectives, and allow you to allocate resources in an organizeed and tactical way. . It would be also serve as an emergency manual when you have to make tough decisions at critical junctures. Much better than develop strategies ad hoc, as the need arises. Sure, you can jot your ideas down on a napkin over lunch and run with it, but these "let's put on a show" ideas are seldom sustainable in the long run. It takes good tried-and-true business practices to succeed in business..
Also, there are a lot of good ideas out there: learn to cherry-pick and adapt to your own needs.
Typical of free idea sources is word-jobber.blogspot.com. While this site limits itself to direct marketing, it's pretty shrewd. Best of all: there's a new idea every Monday, just in time for a long week.