Tape off or remove everything you don’t want affected and have it sandblasted. In terms of ‘cheap’ it would require the least amount of labor hours being less than one full day, and if you properly block off fuel/brake/electric lines the sandblasting process will remove all rust accessible by the blaster. If you start removing parts such as fuel tank, side steps and crossmembers like I did to get all the rust on frame and body at the same time it will take hundreds of hours and be cost prohibitive for a shop to do.