Clowns basically come in three stages. Juveniles are really small and sexless. As they mature, they become male. When in a group, all but the most dominant will remain male, the dominant fish becomes a female. When alone, any single clown will eventually become a female.
How does this impact buying decisions? If you have a chance at a bonded pair that's an obvious mature pair with one male and one female (the female will be significantly larger, and obviously "in charge") then that's probably the most sure bet. Another sure bet is two known-young fish that have been in a group their whole life.
What you don't want to do is buy fish of unknown age or background, or fish that are currently alone in a tank. It's easy to end up with two females this way, which will basically end with one fish dead.
If you have a good LFS with lots of fish to choose from, offering both juveniles in group tanks or well established bonded pairs, there's really no reason to pay more for the pair unless you like the specific fish more for whatever reason.