Any idea of the current going price for tuktuk airport transfers and for a day rate for s21/killing fields?
The Killing Fields area is around 15 kilometres from the centre of Phonm Penh. Most often people visit S21, which is in a suburb of PP, before the Killing Fields so as to have a complete picture of the atrocities at the time. If you do that, then the tuk tuk would cost around $20 - $25.
Tuk-trom Riverside -> Airport ~$8. You may find cheaper, but that's what I'm generally happy to pay. A metered taxi will currently run to ~$10