One of those "What Should I do with my $" Questions

If you have $500 to invest then you might think about simply buying a site from Flippa. Find one with a proven track record of making money, and then just buy it. You can use the profits then to do the same thing.

You can also get some amazingly cheap outsourcing done at fiverr for any of the sites you buy.

Good advice from Orator. I'll just ad that if I were looking to get a good return, I would buy content and drop feed it to my blog.

Will

You don't really say what it is you already know about IM, and in what areas do you have knowledge in - it would be easier to advise this or that (of course within bounds of my own competency) if I knew what do you know already.

For example, if you were fascinated by retro cars, it would be more enjoyable to you to work on a site about them than it would be to work on something you don't care about. You probably already know this.

So I'm just going to assume, that you just want to invest $500 in a site, and not do anything yourself (which, if you don't know all the ins and outs of a particular business, is not the best way in my opinion).

I would just build 3 Adsense sites.

Here's a plan.
Domain: $10x3 = $30
Hosting: $10/month
Keyword research: $10+ per site? Never outsourced this part, but shouldn't be much more. Just make sure its done right, or ask someone that knows this stuff. It is very important to get right = $30
Content: $5 per article (10 articles per site) = $50x3 = $150
Backlinks: $50 per site x 3 = $150

That's $370, and leaves plenty for unexpected expenses.

There are other ways to spend $500, but this is what comes to my head today.

Orator has given you a good idea. I'd try to buy a site that's making money and getting regular organic traffic for a few hundred. Say $150-$300. Then you could use the rest to either further develop that same site with more content and backlinks; or while that site is making you money every month, invest in a system you feel you can follow (be it adsense, amazon, clickbank, cpa, etc.)

Thanks for all the handy advice what I've been doing so far is exploring PPC with my weight loss site. So far its shown minor success and has my mind thinking for scaling up the operations.

I've also bought three domains in the same niche that I'm going to spread out over different IP addresses build PR on, and link together in an obscure way so it doesn't look like a link wheel (hopefully)!

I've just gotta be careful I don't want to get overwhelmed with too many projects.