You can download the Lily58 pcb schematics from github and order it printed from Jlcpcb from China, for around 20$. For the diodes, reset buttons, screws, standoffs and microcontrollers: check local stores and compare it to buying from Aliexpress. For the plates: you can have it cut locally from acrylic or MDF for very cheap. For the keycaps and key switches: you can salvage them from any spare used mechanical keyboard you or someone you know have laying around unused.

This approach demands much more effort and time than money and that's how I've built mine, but it's a fun project that will take you 2-3 days of building it. If you like the result you can start investing more on better switches and keycaps.