The cost for FESS and XprESS surgery under general anaesthesia was based on staff costs for a nurse and surgeon, bed day costs, theatre time, device and surgical consumable costs. The total cost for a FESS surgery under general anaesthesia (including equipment costs of £300) was calculated to be £2,894. The total cost for XprESS surgery (including device costs of £900) was calculated to be £1,884. The equivalent costs under local anaesthesia were calculated by applying a ratio of 0.631 to the surgical costs under general anaesthesia reported in Zilvetti et al. (2009), providing costs for FESS of £1,936 and for XprESS of £1,520. These costs were also used in the model if the patient had a revision surgery.