Remork-motos are sweet little motorcycles with carriages (commonly called tuk tuks around town), and are a nice way for couples to get about Siem Reap, although drivers like to inflate the prices. Trips around town start from US$2, but you'll need to pay more to the edge of town at night. Prices rise when you add more people.
A moto (unmarked motorcycle taxi) with a driver will cost from US$10 per day depending on the destination. Far-flung temples will involve a higher fee. The average cost for a short trip within town is 2000r or so, and around US$1 or more to places strung out along the roads to Angkor or the airport. It is probably best to negotiate in advance as a lot of drivers have got into the habit of overcharging.