Technical Abstract: Body weight is often used as an estimator of production costs in Aculeate Hymenoptera. However, due to differences between sexes in metabolic rates and water content, conversion of provision weight to body weight may differ between males and females. As a result, the cost of producing female progeny may have often been overestimated. We measured provision weight and body weight loss throughout development in a solitary bee, Osmia cornuta (Latreille), to detect potential differences between sexes in food weight/body weight conversion. Male O. cornuta invest a larger proportion of larval weight in cocoon spinning, and presumably have higher metabolic rates than females during the larval period. However, this is compensated by a slightly longer larval period in females. Overall, body weight loss throughout the life cycle does not significantly differ between sexes. As a result, cost production ratios calculated from provision weights and from adult body weights are almost identical. The validity of other weight (cocoon, feces) and linear measures (head width, intertegular span, wing length, cocoon length, and cell length) as estimators of production costs is also discussed. Valid estimators of production costs vary across species due to differences in sex weight ratio, cocoon shape, provision size in reference to cell size, and adult body size.