I don't know the cost of steel in dear old Blighty, but you're looking for RHS (rectangular hollow steel) prices. Here in Australia you could buy an awful lot of it for the equivalent of 600 quid but we dig it out of the ground over here so it might be cheaper! Aluminium on the other hand costs an arm and both legs. I built my machine from steel scavenged off the local tip but that is not the recommended source. Other UK forum members will know more about the likely cost than me.

There are several build logs on the forum which show good designs for welded frames (mine is NOT one of them). If you can already weld, brilliant. If you can't, it's easy enough to do an adequate job and a cheap welder is adequate. You are not building the Forth Bridge and some fairly scrappy looking results will be good enough. You can hide all sorts of sins with a grinder, car body filler and paint. have a look at the pictures in post #5 of the following thread for the proof of that!
http://www.mycncuk.com/threads/13904-Kit-s-Machine

With a steel frame and gantry and a low cost, if temporary, controller you should expect to have most of your immediate budget available for the mechanical components of rails, ballscrews, belts and pulleys etc. and the motors, drivers and power supply plus spindle and VFD.

One very important way you can save money is to consider if you can reduce the cutting area you WANT to build to the cutting area you NEED to build.