I think it's way too much. I'd say 0,2 kg at most. (Unless I'm misunderstanding something.)
Inventory weight of Grenades is 3 pounds.
One pound of explosive and double as much for the casing seems appropriate.
If I recall a typical real-world grenade of the suggested shape of the xcom grenade only weighs near about 1 pound and would have about 0.2Kg of explosive in it. So these grenades are darn-tootin-big like the Mills Bomb and similar defensive grenades from earlier wars (tv trope:they dont build them like they used to).
Another basis for measuring it would be the High Explosive device (110 damage, weight 6 pounds), which is basically a chunk of explosive with a detonator stuck in it.
But in that case id say its a more advanced mix of military grade explosives with a force multiplier on par with nitroglycerin (TNT x 1.5). Otherwise it wouldnt cost so much.
(is there really a game balance associated with such low inventory store costs, I mean when you cant even buy a few demolition packs at $1500 a pop how can you fight a war).
That would bring its firepower up to 9 pounds. And while that's 4Kg not 5Kg (as the prior calculation suggested), the ballpark figures are about the same. Since a difference of (4/5)
1/3 is less than 10% adjustment on the damage itself.
Blaster Launcher Warheads are still on par with being a Hellfire Missile, an Aim-9, and most other air-to-air missiles all by itself.