I have what they call a mid-arm on a frame. It is 12 feet long and you need about 5 feet deep to give yourself minimal room to move around it. I paid $11,000 for a floor model.
You could use the same sewing machine for both regular sewing and long arm-ing but that sucker is too heavy for me to move. If you are looking to spend less, you can go with a sit-down machine like the HQ Sweet 16 or a DSM made specifically for quilters like the Baby Lock Crescendo ~$4000.
Good luck.