Apparently Lexmark has as return program where you save money if you purchase direct and return the used cartridge.
So it is difficult to find toners for this printer for a price below what the official Lexmark supplies site charges.

Lexmark C540, C543, X543
C540A1KG Black Return Program Toner Cartridge 1000
C540A1CG Cyan Return Program Toner Cartridge 1000
C540A1MG Magenta Return Program Toner Cartridge 1000
C540A1YG Yellow Return Program Toner Cartridge 1000
C540H1KG Black High Yield Return Program Toner Cartridge 2500
C540H1CG Cyan High Yield Return Program Toner Cartridge 2000
C540H1MG Magenta High Yield Return Program Toner Cartridge 2000
C540H1YG Yellow High Yield Return Program Toner Cartridge 2000
C540H2KG Black High Yield Toner Cartridge 2500
C540H2CG Cyan High Yield Toner Cartridge 2000
C540H2MG Magenta High Yield Toner Cartridge 2000
C540H2YG Yellow High Yield Toner Cartridge 2000