Photorespiration: energy cost

[Nigel Gomm]

Does anyone know the energy cost in ATP & NADP of returning the CO2 to 
the carbon-fixing cycle (in peroxisomes after the Calvin cycle "misfires" 
by picking up an O2 instead of CO2 and produces the 2-carbon 
phosphoglycolate)?
I.E. What exactly is the inefficiency of photorespiration?