It's very possible I was being daft, but for some reason, I assumed this cost money until I got to that sentence. I guess it's because I still naively assume that any up-front price tag should imply a lack of ads, ergo such a caveat is only necessary if the app is already free.

IMO, they should have listed the price on the page. The other merchandise on their site has prices listed, and an app should be no different.