20the real gate on Shroomite
Chlorophyte you can simply mine. Shroomite Bar needs this bench, the bench needs the Truffle housed and alive, and the Truffle needs a surface Glowing Mushroom biome to move into. That chain takes real time, so start it early: seed mushroom grass on mud somewhere above ground and let it spread while you work through the mechanical bosses.
when he will sell it
Not until Plantera is down, which lines up with Chlorophyte becoming plentiful anyway. The price looks steep in isolation and is irrelevant in practice, because by that point money is not what is holding you back.
it makes more than bars
It also handles the Mini Nuke I and II rockets, which is easy to forget. If you run a launcher, keep the hammer somewhere you actually walk past rather than in a back room, because you will be returning to it every time you need ammo.