In most games, an action of this exact sort -typically called "overwatch" or the like- costs less than a basic attack action with the same weapon.

Why? It's actually very simple, but let's jot it down in point-form.
1. It's a reaction-type skill and cannot be engaged manually. Enemy units have control over whether it even occurs at all, since they can fire without moving, take cover while moving or even step behind allied units; causing friendly fire in some cases.
2. It's meant to provide an incentive for allied units to group up, but maintain a certain distance so they can operate as an effective squad and lock down enemies to prevent your defensive line being broken.
3. It's a skill meant to be used at the end of a turn when you've expended all other forms of action and have only a small amount of points left to spend.
4. Typically skills of this sort hamper enemy movement when engaged, regardless of whether they land a hit. Again, these sorts of mechanics are meant to make it an effective tactic and skill for use in holding the line.

Furthermore, because the accuracy of this skill is typically lacking and it has no special boosts to attack power, it's absurd to give it a higher cost than a basic attack with the same weapon given this game also has ammo consumption and a somewhat unforgiving friendly-fire issue with some weapons capable of stripping allied armor with a mere accidental splash of bullets.