Wow, a question about a 7 year old club after a 7 year layoff. I'm not sure how good of a deal the price is, but the technology is so different after all that time. Moving up from 2003 to 2006 won't be as much of a game changer as finding even a 2010 F2/F3. Just a quick check looks like the 910 can be had for under $50 and the 909 is under $40. If I liked the 975 I'd keep it for now. I have a 910 7W and I'd still be more likely to part with my wife than that club. Because it does what I ask it to. But the technology is closer to the 915 than to the 906. In 6 years, if I still have the 910, I wouldn't even consider a 913. Or leaving my wife.

Mike:  I checked in the PGA.com Value Guide which is used to determine the value of a club and its trade in value.  The 906F4 was released in 2007 and has a 431 head.  The resale value on a 906F4 runs $22-$31 dollars.  The trade in value is about $12 if they were to trade it in on a newer club.  It gave minimum info on your other club and it was released in 1999.  This is the blue book of golf clubs.  Hope this helps! 

Chuck