You can check the reported sales at DNJournal, Sedo, Afternic for sales data. I don't recall seeing anything in the way of a trend. There are always going to be some sales following big successes of other My(Word) websites. Call it the HotMail effect or, in this case, the MySpace effect.
In this "domain space" I don't see the value (price) in the domain but, instead, the value/price "is in" the obsession of the acquirer.
"We/I must have that domain! It's the only/best one for this project!"
So, I don't really see precedential value in prior sales.
This version of domaining has been around for years. One famous example was a person who "cornered the market" (and spent his fortune) on 247(Word).com domains, the logic being that the Web was going to make the world a 24-7-365 place and therefore everyone would naturally see 247(Word).com domains . . . as something.
It didn't quite work out. And, this approach has been replicated ad nauseum.
YouTube? (Word)Tube or Tube(Word) everything.
It's the lazy man's version of greasing the rails to success: "MySpace IS a big success therefore, if I start a site called MyBathtubRings.tld people will . . . whatever".
I wouldn't pay a premium based on the notion that a My(Word).com domain is going to drive value and drive success. Just as people think MyWord domains are the new "it" MySpace will start to show signs of its decline and My(Word) domains will suffer by association with the bust.
YMMV. Good luck. I'd use the extra cash on some software improvements.
P.S. I didn't and don't mean to call you "lazy", for all I know you're the hardest working and most imaginative man or woman on the planet. However, I do mean to challenge the thought that spending for a My(Word) domain will be a better use of funds than using the same funds to improve something else about the website. It's your money, though, and who knows. Maybe you have enough to do it all. ;0) Hope so. I'd sure like to be so positioned.
[edited by: Webwork at 1:19 pm (utc) on Feb. 8, 2008]