Options for lower budget Machines include:

Steel box section as while weight increases which makes moving the beastie a problem the rigidity also increases which aids in accuracy.
Down sides is that it requires very accurate cutting (but that is a common to all materials); it is required to be coated with something to stop oxidation from getting a start; may require welding which then brings it's own problems.
Up sides is the weight it brings for a base helps decrease vibration massively; has a low expansion rate to temperature changes (not non just low); With a little bit of looking and shopping around can be gotten for a lot less then profile or Ali' box section.

Recycled Profile, IF you can find some one or some where that deals in break down of machines that are past their useful lifespan you can get good profile on a discount. Upside it is profile which is light compared to steel and yet retains the rigidity needed for repeatability/accuracy.
Down side is that it will most likely have some work hardening in the section and the will be drill marks and such. Which mean you have to be a bit careful in choosing what you want to work with.

Also starting with an air cooled spindle like a Kress or something a bit better (again look around for folks selling on equipment in good condition). That or save up and get a water cooled unit though realize a good a good VFD is not cheap nor the proper wiring and in line filters to have it work at it's best.

Depending on what you are doing with it Trapezoid Screws with spring loaded anti backlash nuts can be as accurate as you might need. This is where a serious sit down and looking at what you are going to be doing in real world here and now terms can help massively. It is why my second machine that Jazz is building is custom and built towards a goal of specific usage. The frame and structure of a machine is from what I have experienced only about 40 to 50% of the overall cost. Steppers/Servos, drivers, BOB, computer, spindle, vacuum for cleaning, and many more add up quickly and from personal experience are equally important.

Another option entirely is start haunting the equipment auctions and chatting with a few folks here who know where there are auctions of equipment. Go and look a few times and get a feel of what you are looking for. See if one the folks here is heading to one that you can go with. You would be surprised what you can get used industrial machines for at times. Depending on what you plan on doing, refurbishing an older machine might give you an affordable answer to your goal. Yes most of them are heavy as heck and will need work on them. Look around and talk to folks here and at model engineer swap meets and other places the similar minded people meet (robot war groups come to mind) and you might fins what you want exactly or so close that it does not matter.

Good luck and keep asking questions.

Michael