Ask HN: How to gauge the value of data

For the past few years I've been running a website to provide free access to statistics on hundreds of thousands of YouTube channels (http://socialblade.com/youtube). The site has been growing like crazy and currently serves up about 6 million page views a month. This data I've collected from polling YouTube's API (they only give current data not historical) and then storing the data as well as some of my own 'magic' as well for some calculated stats.

I still plan on providing this for free to anyone that wants to look up their stats or stats on any given user but every now and then (more and more often) I'm contacted by companies wanting to purchase/license some of my historical data as its not available anywhere else. Thus far I've avoided giving out too much data and just given things away for free but I know I'm selling myself short.

I'm interested in working with these companies as I'd really like to see if I can turn my once hobby into a real company but I'm really not sure how much I should charge for things like this. In providing this data I'd be giving out a lot of why my site is valuable in the first place so I know I'll need to make sure there are legal proceedings around it to make sure they don't turn around and resell it themselves or duplicate my site. Much of the data also is available for free on the site itself as I mentioned but not in a nice exported, sorted format. In many ways I have a lot more data then big data mining companies like comscore have and I know they're making a killing off of selling their data.

This is why I'm posting here. This is really my first experience in running a startup of sorts. I have a lot of valuable data I just am looking for any sort of assistance in gauging on what that value is. How do I know how much to charge without scaring away a potential client with an out of the park number but also not way undersell myself either? Any help would be greatly appreciated! Thanks!