How much of a performance increase you'll see depends on what you're running -- the more CPU intensive the applications, the more significant the increase.  
Hello, I currently have a Latitude E6520 with an i5 2520M CPU. Considering their low price (>$30 on eBay) I was thinking of upgrading to an i7 2620M. How much of a performance difference would there even be between the two CPUs? I'm not really in need of much more power with the Latitude, but if I can get a noticeable performance increase for $30, I'd be happy. Also, it does have 8GB of DDR3 RAM, and a SSD.
Upgrade to i7?