We've traveled through Cambodia for a couple of days on motorbikes. That's a way I can highly recommend to everyone! You'll see another site of the country and will have a great experience and if you like, you'll also find some adventure!
There are two rental stations in Phnom Penh which are situated right next to each other. The 'New New Motocycles' & the 'Lucky Lucky' rental station. It's easy to bargain there. You just have to visit both shops and ask for the prices... We paid for a Honda Baja (base model is the Honda XL) 250ccm with a big fuel tank 8$ per day. I guess it could be around 10$ now, but that's still a great deal.
But be careful: There will be more costs to pay while you are on your trip for several days. The bikes aren't in a 'new' condition and everything else then perfect and you should have a test ride first, before renting them. You don't have to be a mechanic to test the simple stuff... You should at least test all gears, breaks, lights & horn.
It's no problem to let smaller repairs be done on your trip. In each you'll find someone who is able to do these and also the costs won't hurt you. I've lost the key with a running motor and needed a new one and also a new closure for the fuel tank. We arrived in the evening in town and I wanted it be fixed the same night. That cost me 8$. If I would have waited to the next day, I would have paid 5$. You see, that was a fair deal.