They cost $1.00 for a bag of 5 ball bearings of one size. If you have not ordered anything from Micro-Tools, think of considering ordering needed stuff that can help you a long way especially in sensor dust cleaning:
Large size Ghiotoos Rocket BLower
Wet Sensor Cleaning tool kit
Thanks,
Hin
I'd also add to that list a small magnifying loupe/flashlight thingy, from micro-tools. It's a pain guessing where dust is on your sensor and checking after every puff by shooting at small aperture. That lets you see dust on your sensor, to an extent.