Take a look at my blog post. I wrote about something similar to the question you have. Take a read. http://www.abeltravis.com/1000-2/
I would suggest that you invest in the field you like and mastered...or if you would like to invest in a new market, please study it first and know what it takes to make money back from that market... Internet Marketing is a vast world..it can bring you fortune or it will bring you disaster. So better know what you want to do, do some research on it and take it to action !!
I wouldn't recommend buying an established website with revenue since in most cases, the goal of the seller is to make some quick cash and you might lose the revenue quickly. Invest in a good e-book instead!
The real answer is you don't invest it. When something pops up that you really have an ambition for, you will enjoy working on the project and have a better chance of success and your money will become much more valuable for you when you will need it as opposed to spending it on someone elses behalf and you'll end up losing the money. As much as you might think $500 is nothing, it's enough to start 10 businesses and become a millionaire. I've made hundreds of thousands of dollars from nothing, it's just about having the ambition of becoming into business and having that 'entrepreneurial' spirit that has made them millionaires ten times over.
That would also depend on whether you want to actually work in what you invest (e.g. IM tools, content, etc) or just earn $$$ passively. If you are thinking of starting a business, see where your core competitive advantages are before you do anything else (e.g. copywriting can pay very very well and you'll find loads of sites advertising courses in that field but if writing is not your thing, you'll want to invest your money and time in something you'll thoroughly enjoy and be better than most at).
invest in website with minimum 100$/month and after 6 months maybe you will have revenue, is just my opinion.
You need to buy a website which is earning good. And then manage it for some months try increase it's revenue and then sell it back. In simple language do Flipping.