How does the mint mark on a half dollar effect its price?

That's AFFECT, not EFFECT. EFFECT is normally a noun.

The mint mark on ANY coin may affect its value because it lets collectors know where the coin was minted, and thus indirectly how rare it is. In most cases up to about 1940, fewer coins were made at branch mints than at Philadelphia, so they tend to be worth more. For example, in 1909 14 million Indian head cents were made at Philadelphia but only about 300,000 were struck at San Francisco, so the latter are worth far more.