Bullets indeed do essentially nothing against weather balloon type craft. Bullets from ground level, even less so unless they're significantly larger rounds. Oh, and you don't want to be downrange of all those misses either.
Flak would probably be the "cheapest best bet" for ensuring damage to the balloons' payload, but the collateral damage from debris could be significant. They will probably use some small missile to do the job, which is the only way to be sure, really. However, that's not ideal from a cost ratio standpoint, which leaves them wide open to the fact that;
Those balloons are also probably cheaper than you estimate, when produced at the same scale as the rounds of which you speak.
Which means that when and if the Russians put something in place to attempt to stop the balloons, the Ukrainians can simply send arse-loads of decoy balloons to waste Russian ammo supplies.