Threema is a better comparison. They offer a commercial, ad-free product with E2E for a one-time fee of 3.99€ in the iOS app store. They also don't sell your address book or other data. They don't even have your phone number.
If that's the case, any price is a losing proposition - since you're competing with free. Users don't do business-case analysis, they barely compare features. I'm not sure the Signal features can easily justify $5 over FB/WA/Insta.
Threema is a better comparison. They offer a commercial, ad-free product with E2E for a one-time fee of 3.99€ in the iOS app store. They also don't sell your address book or other data. They don't even have your phone number.