If he was over taking the line of traffic on the right there are 3 possible scenarios:
1) You checked in your mirror, signaled, did a check in your door mirror, before turning. The motorcyclist saw you indication and decided to overtake anyway, so 80% motorcyclist fault, 20% yours for not seeing him in mirror checks.
2) Mirror signal turn. At least 60% to 80% your fault, depending upon where the motorcyclist was and how long you signaled before turning as you didn't see him and may well of left him with no where to go.
3) Signal turn. 100% your fault, where you didn't see the motorcyclist and left him with no where to go.
The lesson is to ALWAYS have a final look in your door mirror (it is called a suicide check, where an idiot tries to overtake, while you are signaling right) before turning right. Another tip is always when maneuvering to turn right make sure your front wheels are straight while waiting to turn, so if your car is rear ended you are not pushed into oncoming traffic.