I have just purchase a license for my Mac, do I need to pay for the Pro version on my iPhone as well?
1Password Version: 5.4.1
Extension Version: Not Provided
OS Version: 10.10.5
Sync Type: Dropbox
@AndrewCurran the first thing to make clear is that 1P for Mac and 1P for iOS are completely separate products. 1Password is licensed per person and per platform, rather than per device. You can read more about AgileBits' approach to software licensing in this knowledge base article.
That means that if you want the pro features of 1P for iOS you have to make the in-app purchase.
Hopefully the information Stephen has provided proves useful. The base 1Password for iOS app is completely free, but if you would like the Pro features, as Stephen mentioned, those are a separate purchase.