Well, purple is not all that generally common bred with breeders. i guess they yet havent established a good pure purple betta yet! but at the pet store, it is no that common yet not that uncommon also.
since you said he was a pet store betta, breeding him with another purple pet store betta, the young may not end up being purple. they could be green, blue, red, yellow, orange, whatever. this is because pet store bettas, since htey were shipped to the petstore, there is no way of knowning which genes they hold. I currently have a female purple betta with red fins, i just need to find her a purple male with red fins, then they can breed. she is already collecting eggs...