I volunteered for a research group that did amateur balloon launches like this in college. Our payloads were about 8 pounds and were regularly sent to 30k meters/100k feet before a cutdown command was issued. I don't remember ever measuring how far the package would get from the launch site when it reached maximum altitude, but the package could pretty easily travel 100 miles from its launch point to the touchdown location.
Here's the site if you want to learn more - they've got some neat flight data posted if you're interested.
