* There is an economic impact (lost revenue for firearm producers, sellers, resellers, ammunition producers, gun ranges, sport associations, hunting groups, accessory producers, etc).
* There is a personal utility cost (enthusiasts, hunters, and sport shooters can't enjoy their hobbies anymore).
* There is a communal cost (those people now have fewer reasons to come together and form groups).
* There is a personal defense cost (citizens and especially at risk groups are less able to defend themselves).
* There is a national defense cost (armed citizens impede invasion).
* There may be a military readiness and recruitment cost.
* There is an increased probability of tyrannical government (armed citizens impede this, I don't know to what degree).
* There are time and legal costs (this will be fought aggressively and take up large amounts of legislator and judicial time which could be spent on other matters). In the US this would require a full constitutional amendment which is by design a non-trivial undertaking.
* There are regulatory costs (someone needs to define and interpret what bans mean).
* There are enforcement costs (all existing firearms would be grandfathered in. To my knowledge there is no legal basis for eminent domain of firearms or seizure of assets that were legally purchased pre-ban. For example, any alcohol purchased before prohibition took effect was still legal to possess and use. Any automatic weapon purchased before the ban on automatic weapons is still legal).
You can argue that it's still a net benefit to ban firearms (and you may be right) but I disagree that there are negligible costs.