I paid about $150 to have the frame I built bead-blasted and powder coated. It would have been less, but $150 was the shop's minimum charge. There were other places in the area that had a $100 minimum, but they didn't really seem to know anything about bikes. I don't know of any powder coat shop that will disassemble your bike for you. Expect this to add $50-100 to the cost if you get your LBS to do the work. If you want a custom-order color, you may have to pay even more.

Your frame will need to be free of all paint and other contaminants before it can be powder coated. Most powder coaters will take care of this for you. If a shop tells you that bead blasting isn't necessary, go elsewhere. Be aware that modern bike frames are made of very thin tubing (e.g. 0.5mm or 0.020 inches). Sand blasting is too aggressive for use on a frame. The process typically used is called "bead blasting", though I have no idea if glass beads are still the material of choice. I've also heard of walnut shells and baking powder being used.

You'll want to make sure that your powder coater properly masks your frame before it is powder coated. They need to plug all of the tubes as well as the bottom bracket. Anything with threads on it needs to be plugged (bottom bracket, water bottle bosses, rack mounts, derailleur hanger, etc). The guys who did my bike also laid small circular masks over the areas where the quick-release skewers for the wheels seat. They claim that pressure from clamping the wheel can cause the powder coat to crack! Don't know if it's necessary, but it sounds (somewhat) reasonable to me.