Yeah, I noticed that too. This was a problem I had playing my Commando in Star Wars Galaxies way back when. They were one of the few ammo based classes, at least 3x as expensive to run as the other combat classes. We noticed it quickly with a Bombard soldier in the group, though in play it hasn't mattered much since we've been able to loot enough grenades.
I would say 5 per box is probably an ok adjustment. 10 per may actually make them too cheap. But they definitely need something.
I had thought that grenade prices were too high. But I think that I was wrong, what makes me think this is that all gear in starfinder is in practise disposable so you lose 90% of the cost of all your gear every few levels as you can only sell for 10% of cost and cannot upgrade (unlike pathfinder were you can sell for half and upgrade most weapons and armour) this means the inflow of cash to maintain WBL is going to have to be higher and so completely replacing all your grenades is a cost comparable to completely replacing your weapon.
They're supposed to be an expensive, non-viable (for constant use) option.
You're not supposed to use them multiple times a fight, or even for every fight. Think of it just like you would think of any of the damaging consumables from Pathfinder. How often did people use wands of fireball? Not very often because the save was low, it didn't do a lot of damage, and the price was very high for what it did.
In Starfinder the whole idea is to find grenades and use them for particularly difficult situations, not constantly use them. You're also probably not intended to go out and buy lots of them.