I would like to find out the unique monthly traffic for a competitors subdomain. I know that compete.com does this but it is very expensive. Are there any free or less expensive tools out there that I can use?
There really is no such thing as reliable, public-facing analytics data for websites. Some websites choose to make certain statistics pubic from their own private analytics, but these numbers can be hard to verify.
There are several companies that claim to show public analytics data for any website on the internet, but these aren't actually capturing real traffic data. Instead, they're tracking a small sample size of internet users, looking at what sites those people visit, and then extrapolating that data out to generate trends for the whole internet. However, I've seen these numbers be orders of magnitude off from actual traffic data, and they can be easy to tamper with.
Two companies that jump to mind that offer these sorts of public-facing analytics are Alexa and Compete. Both of these companies will let you lookup any website to see what its traffic is like, but keep in mind that this is often inaccurate.
There are other companies like Quantcast that allow a website owner to embed a tracking code on their website. Quantcast then actually tracks every visit to that website. Quantcast then gives the site owner very rich, detailed analytics, and also offers a more limited subset of that data for the public to view. If you're looking for a Fortune 500 company, there's a good chance they'll have Quantcast code installed on their website, so you could try checking there.
Calculating accurate data for competitors: If you're looking for a relative comparison (ie, your site compared to a competitor's site) there's a quick calculation you can perform to get a fairly good idea about your competitor's traffic:
This will give you a fairly good approximation for how much traffic your competitor is getting.
Instead of paying for such tools that are not 100% accurate always; why don’t you try Google search? Just enter the company/product name of your competitor and get to know all the external and 3rd party websites of all sorts (blogs, discussions, videos, News and PR) where your competitor have created a link or some blogger has reviewed his company/product. The referring websites returned by Google search results are pretty accurate, relevant and FREE as well. Try it, it works; really!