Yep, training Guardsmen costs way more then giving him few more spare mags.
Lets take a math class - spare mag for lasgun is 15 thrones (and that is retail price, I am sure Guard would get them cheaper - ~10-12).
Even for poor 3-month training Guardsmen would eat more then that amount of thrones in food, never mind all equipment that he will be issued and which is generally lost in case he dies (fatigues, boots, backpack, etc)
So, unless there is major shortage, if he has a lasgun he will have plenty of mags for it.
As Boss Gitsmasha noted, PDF would be lucky to have anything, but again, if they have Lasgun they will have reasonable amount of ammo, especially considering how it can be easily charged - so ammo shortage is much less likely then with SP weapons.