Transferring numbers to new worksheet automatically

Hello, I'm making a spreadsheet showing weekly costs. I've set it up so that each worksheet is a week, with totals using =SUM. I want to transfer the weekly total (as a number, not a formula) to the next week as carried forward but when I copy and paste, the numbers don't appear, just #REF. I used to study A level ICT and I'm sure I vaguely remember a simple way to do this, but can't figure it out. Help!