Aluminium is expensive and I think my build must have been a few hundred pounds. Mostly ecocast as it is worth the extra not to have to machine it flat yourself (assuming you have the ability to do that in the first place given you are building a milling machine)

If you need something that big maybe go for heavy gauge steel section and make a welded structure, then plate over with thin steel panels. Steel is cheaper, 3 times stiffer but needs a different build process.