As for that trade - yuck. Shawn Marion is not a top tier talent. And who's your shooting guard? If we trade Ray, it'd better be for somebody a hell of a lot better than Shawn Marion.
Please see below.
Just to be clear, the point of the post is not the suggested trade above, which was for illustration purposes only (in fact, I wouldn't do the trade if I were Boston). The point of the post is to think seriously about financial constraints and ways the Cs might improve while not adding much to the team salary.

Let me try another: Ray Allen + Scal + TA to Houston for TMac + Ron Artest (S&T). Again, not sure I would do it. I guess the problem is that Ray's contract is so big it is hard to find workable deals.... but the whole point of this exercise was to raise the possibility.
I get you now, sorry I missed that part (though you still called him a "top tier talent" and I stand by disagreeing with that).
As for the last part, I don't think the problem is that Ray's contract is so big. The problem with trading Ray, to me, is that he's not just a big expiring contract like Zydrunas Ilgauskas or Ben Wallace or Jermaine O'Neal or Larry Hughes. He's a big expiring contract who's an All Star and essential starter on one of the five best teams in the NBA, so we have to replace the value he gives us. But he doesn't give the same value to most other teams because they're not going to be competitive in the short term so what do they need with Ray Allen?
For another team to give up a max player who's better than Ray (and if you're giving up Ray, don't you want to get better in return?), they want some young piece of value in return. I don't think Vince is better than Ray, but New Jersey wanted some young piece - Courtney Lee - in addition to expiring contracts. So they would've wanted Ray and somebody like Perk to give up Carter. We're already giving up a key starter whose production we have to replace - we don't want to give up anything else.
For example, Milwaukee's in rebuilding mode. Let's say they're willing to give up Michael Redd. Getting Ray back for them is just about expiring money. Not only will they want you to take on another contract (Redd has a player option next summer and expires in 2011 anyway so it's not a ton of savings) like Gadzuric ($14 mil for the next two years) but they want a good prospect for their younger (though turning 30 this summer) All Star caliber shooting guard. Given that Redd might replace Ray's stats, but also might be injured for the entire season, and we add on a whole bunch of salary in Gadzuric, why would we give up a guy like Perk (which is what I bet Milwaukee would want) too? We downgrade at center and are rolling the dice at 2 guard and paying a whole bunch of money to do it. Why?
Most reasonable trades involving Ray unfold along those lines. To make it attractive to the other team, we have to give them young talent. Our young talent consists solely of Perk and Rondo. So we'd have to get back enough to replace two starters, and other teams are not giving up that kind of quality for a package that is, in their eyes, one young non-All Star player and expiring money. It just doesn't work out.
That's why the most logical thing to do with Ray, to me, is to find out if he'd take a 2 or 3 year deal at about $10 mil a year. If he would (and I think that's a fair price) then you get 2-3 more good years out of Ray, then in the summer of 2011 or 2012, he's a $10.5-11 mil expiring contract to be included in a trade but at that point, you're not trying to replace his production anymore. You're only using him for salary space.
I understand the whole trade him while he still has more playing value argument, but let's face the facts. Right now, this is a league with about 6 teams willing to pay for talent (Bos, Cle, Orl, LAL, SA, Dal). There are another half dozen or so teams that are looking to improve, but not giving away assets (Den, Hou, Wash, Det, Port, Atl, Uta) and a whole bunch of teams just selling. To those selling teams, they're not looking to be competitive in the next 2-3 years, so to them, Ray Allen is just expiring money and they want some young assets. We're not gonna deal Ray to one of the 5 other buyer teams, so it really comes down to those half dozen or so neutral teams who might give you value for him. But they're certainly not handing away players for financial reasons (i.e., Detroit laughing off Ray and Rondo for Stuckey, Rip, Prince, even when that arguably doesn't help us).