and does anyone know what sort of power it will produce and at what sort of cost?

yes, l did it to my 306 gti6 and got an extra 12hp , you will probably get close to 200hp after for around 3to500 pounds

yes u can get more power from ur engine by remapping but u should take into consideration the reason the manufacturer uses the the original map its to give u reasonable performance coupled with acceptable economy & drivability IE lumpy tick over , having to use low gears all the time around town & in traffic it will alter the power band of the engine IE when max bhp & torque kick in the cost varies on which stage of performance u want to achieve also it will affect engine life & any warranty is u may have so u should think carefully before u spend ur hard earned cash on this type of tunning hope this helps you