I don't think there's any rule. It's not even what the pipe major says, it's what whoever was ordering the kilts when the band started out that controls, since you want everyone to match. 
While I've played with bands that were just starting up that required I buy my own kilt, most bands issue kilts to their members. That's why you play parades and other gigs, to raise money for uniforms and reeds and the other sundry things that bands need (Beer, for example! Or root beer, in the case of your junior band. ;) )
I would suggest that the band play some gigs in whatever they have available (or just put everyone in black trousers) while you play some gigs, take donations, and otherwise make money to buy kilts, then invest in a band stock of 16oz kilts, which will last a very long time, even with pipe band wear.
I don't know that band kilts are a place to cut corners.
"To the make of a piper go seven years of his own learning, and seven generations before. At the end of his seven years one born to it will stand at the start of knowledge, and leaning a fond ear to the drone he may have parley with old folks of old affairs." - Neil Munro
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