I know some polish sites do offer this option but it's a bit expensive as linens expensive as hell. Cottons another option but I dont think thats breaths as well as linen does.
Linen is pretty cheap. It's about $7-$10 a yard, nice wool fabrics are double to triple that. You're not paying for the linen when you go to those sites, you're paying for a custom fit garment that's properly shaped and quilted. The real expensive stuff is the hand constructed garments. It's the labor and proper construction that's making them expensive, not the materials. Cotton breathes just fine, but cotton fabric wouldn't be appropriate if you're going for a historical gambeson for your time period.
It's been a couple years but I think cotton was $3/yd when I was at JoAnn's last, so $7/yd is more than double. I tried a muslin prototype, at $1/yd, but it didn't go well, and I gave up. Relatively speaking, linen is expensive compared with cotton.
But I completely agree on the primary part of the cost being the custom cut & labor aspect.
A Revival arming coat, linen, two layers, no padding, is $250, and that's a "pick from a couple standard sizes" off-the-peg price. Historic Enterprises is even more expensive.