It won't be $300,because i think nintendo said something about not having anything priced over $200,ftw.
Nintendo Wii was $250 when it was first released. The DSi was $180, cutting it close.
The 3DS is confirmed to be 25,000yen for Japanese, which is roughly $300 for us. That is excluding some other forms of costs/fees and whatnot, which will possibly make the 3DS a bit lower when it reaches western markets.
Though it may not be exactly $300 on release, it will be pretty damn close. I am quite certain of that.