Audit Commission
Costing Care Pathways: Understanding the cost of the diabetes care pathway - 16 page PDF guide and 1mb "Diabetes cost comparison tool" spreadsheet which suggests that the "average annual cost per patient with diabetes (based on their analysis) is about £380."
(8 January 2013 - page has moved to archive-audit: http://archive.audit-commission.gov.uk/auditcommission/subwebs/publications/studies/studyPDF/3690.pdf)
YHPHO
Diabetes Data Directory lets you answer the question "What is the cost of the diabetes service?" among other questions.