There doesn't need to be any justification. If that's what OP wants to charge then that's reason enough.
That being said, OP should probably realize a lot of people don't pay for software--even in HN.
That's why OP needs to make sure the users are the product and find some way to sell the user data to advertisers.
OP should contact restaurants and allow them to place ads in the recommendations. He should also sell access to user data and allow restaurants and advertisers to target free users.
He can have a subscription tier that gives you privacy.
That being said, OP should probably realize a lot of people don't pay for software--even in HN.
That's why OP needs to make sure the users are the product and find some way to sell the user data to advertisers.
OP should contact restaurants and allow them to place ads in the recommendations. He should also sell access to user data and allow restaurants and advertisers to target free users.
He can have a subscription tier that gives you privacy.