June 28, 2017 17:54 Updated Follow There are several things that factor into the final cost of extra service. We suggest you read Buying Extra Service Credit if you are a member of PERF or TRF. For other funds, . Related articles What is service credit? Can I buy extra service credit in PERF? Can I buy extra service credit in the Teachers' Retirement Fund? How can I estimate a monthly annuity benefit? How do I change the personal information INPRS has on file for me?