Together with the iPad version of the original game (also previously $9.99), its price has been dropped to $4.99.
The iPhone version of Real Racing launched at $9.99 back in June 2009, maintaining that price until November. It's also had a price cut down to $1.99.
Firemint says these prices will run until February 14.
As for the reason, you can see from the AppAnnie graph below, since its launch in December, Real Racing 2's top grossing ranking has been declining at an increasing rate.

Price vs demand
As proven by analysis of the pricing strategies of Gameloft and EA Mobile, $9.99 is a difficult price to maintain.
While both publishers have launched games at that price - indeed EA going higher on occasions - over time $4.99 and $6.99 seem to be the sweet spots for longterm sales.
At the time of analysis, Gameloft didn't have any games more expensive than $6.99 despite launching over 25 at $9.99.
[source: Firemint blog]