how can i stop the pipes blowing off my n reg mondeo when accelerator is pr?

N Reg Mondeo 2 litre, 100500 miles.
When i accelerate i lose power and slow down, lift the bonnet and a small pipe(just off centreon the right side as you look under the bonnet) has come off, push it back on and all is ok for a short while.
Whats causing this and is it cheap to fix?