I'm not a heavyweight builder but when we costed up machine even a basic pusher was 1.5K. Thats mag drive, lipos, esc's Hardox, no welding or cutting costs included.
You need a design really. I believe Eruption was 3K on its first build. Others will probably be more. Most people will weld the robots themselves, have the pieces plasma or waterjet cut by a company. Then you factor in the cost of the steel. You really need hardox but you could do a first version in mild to test the design.
We figured you need around £3000 to build and maintain a flipper for a few events then spend more to keep it running. But that is us. Your way may make things cost less or more.