2 rockets to destroy a Metal Door. That's 2,800 sulfur in raw materials.
1 C4 charges to break a Metal Door — roughly 2,200 sulfur plus metal fragments and tech trash.
4 satchels to destroy a Metal Door in Rust. Satchels have the best damage-per-sulfur ratio on soft sides.
The cheapest explosive option for a Metal Door is 18× Beancan Grenade at roughly 1,080 sulfur.