The most costeffective approach to constructing LEGO mills involves knowing what bricks are cheapest, this is what they are.

2x2 Brick: https://www.lego.com/enus/pickandb...
16x16 Plate: https://www.lego.com/enus/pickandb...
Baseplate (on sale) https://amzn.to/3tEN3Uh)
1x4 Technic Brick: https://www.lego.com/enus/pickandb...
1x4 Technic Brick: https://www.lego.com/enus/pickandb...

Other shopping option, I mainly use for used bricks: BrickLink.com

Spreadsheet: https://docs.google.com/spreadsheets/...

Merch: https://www.etsy.com/shop/TimsLittleShop