As I will be going to poipet via Siem Reap soon, I search the internet to find the taxi fare from siem reap to poipet border and after making some enquiries, I found that the cost is usd45 per trip. The journey takes about 2.5 hrs.
Grab is also operating in Cambodia but the fare from Siem Reap airport to poipet border cost 452,500 riels (usd113)
Unfortunately I still don't know where to rent a motorcycle in poipet.