Here are some articles estimating LUE in winter wheat. Unfortunately, I only have direct access to one of the articles so I could not give you a range of values.

http://www.sciencedirect.com/science/article/pii/0168192388900160

http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2486.2002.00503.x/pdf

http://www.tandfonline.com/doi/abs/10.1080/01431169008955130#.UjxZE20hzms

During field trials I've conducted in Mediterranean environment (eastern Sicily 37° Lat N , 10 m a.s.l.) observing two cultivar commonly adopted (Creso and Simeto), I calculated values ranging between 1.49 and 2.09 (g/MJ) relating total epigeic dry biomass and cumulated photosynthetic active radiation (PAR)

what about spring wheat? Seems to be higher

It's about 2.2 g/Mj.