a commonly used 'budget' case is often an old computer case, especially the ones with quick detach sides and a clean front panel, usually from an older server or router...
lots of room, easy to work on, and usually real cheap too..
depends where your putting it, if your mill is enclosed, and its located somewhere dry and relatively clean then an old pc case should be ideal. If the box is likely to be near metal chips or dampness (coolant) then most computer cases dont offer much protection.
The next step up would be a rackmount chassis, which can be found reasonably cheaply if you shop around, and are a bit more rugged. They are not watertight, but only have ventillation at the back, so are reasonably good at keeping chips and the odd bit of coolant out. They are around the $50 mark new, and can be found cheaper on ebay from time to time.
The best solution is an IP rated enclosure, as these are designed specifically to keep dirt and moisture out, so even if the garage roof leaks the kit is ok. They are very expensive, but do come up on ebay quite a lot, i managed to get a stainless steel ip66 enclosure for around $80 which was 50cmx40cmx20cm, and will hold all my gear.