A major challenge with commercialising enzymatic processes for producing lignocellulosic sugars is reducing the production cost to commercially-acceptable levels. A commonly-held assumption is that higher sugar yields drive cost reductions, but high yields often come at a trade-off with other factors. Here we consider the enzymatic hydrolysis of a thermomechanically-pretreated Pinus radiata and evaluate the impact of adding polyethylene glycol-4000 (PEG) on the sugar yield. We use techno-economic modelling to determine the benefit of PEG addition and the technical conditions that should be used in the enzyme hydrolysis stage to give the lowest sugar production cost. Our results show that adding 0.05-0.10% w/v PEG significantly enhances the substrate digestibility, with glucose yield enhancements of up to 30% at lower enzyme doses. Adding 0.05% PEG reduces the minimum sugar production cost by (similar to)10%. Increasing the yield in the enzyme hydrolysis stage generally reduces overall costs, but the lowest cost conditions (5 or 10 FPU enzyme/g substrate for 24 h) are not necessarily those affording the highest sugar yields. Thus, when optimising enzymatic hydrolysis, the benefits of increasing sugar yields must be balanced against the higher costs of additional enzymes or the higher capital and operating costs associated with longer hydrolysis times.