when you have to get 64 rot and wait 20 in game days just for 1 block of terra preta i feel like it's fair to want to get more out of it.
how expensive it is entirely depends on how much saltpeter/bony soil you need, also unless someone is immediately trying to get terra preta at the start of the game they probably either have some saltpeter or know where some is, and yes saltpeter is used for other things but the point of this is the choice between looking for high fertility soil and crafting it, so if saltpeter is only used for fertilizing and making better soil then that choice would be boring.