I posted this back in March:
It's hard to say just how much the vehicle can be bought out for.
A few of my friends have been able to deal with GMAC and bought out their vehicle for less than the stated buy out price.
One of them actually went to another dealer than the one they got the vehicle from because GM actually owns the vehicle and not the Dealer.
Myself, my lease is up at years end, and when I talked with my Finance person at the dealer ship,..
he told me GM rather not take the vehicle back unless that have to.
They'd would rather sell it to the leaser or give them money towards a new vehcile buy.
The next time I was at the dealer I talked to the manager.
He said "no way will GMAC negotiate."
Then when I told him I could buy the same as I have now for less than my buyout, .... he changed his story:
Said they, the dealer, .. could buy the vehicle from GM and then sell it to me for less than the buyout price

It also depends on other factors.
If the dealer has someone waiting in the wing for a nice low mileage vehicle etc, coming off lease, they won't negotiate as much because they know they have that vehicle sold as soon as you turn it in and most likely for more than you want to pay.
The other situation is GM has to send them to the auctions and they'll get next to nothing for them all things considered.
In my opinion, at this point in time, the only way to know how low a vehicle can be had for when coming of lease is to push for the lowest price and see what happens.
Every dealer and situation is slightly different and won't know until you try.
..
Here's an update since then:
I found a new Dealer who does Excellent business with no BS on pricing for their vehicles.
I asked about my Buy out for my EXT and they told me the contract is Binding ... But,..
the dealer could take the vehicle back and it would stay at the dealers.
I then would tell them what I would like to pay to buy it back.
The vehicle then goes on the Auction block,.. through the Interent !
Now the Dealer would bid on it up to what I think I'd like to pay to buy it back.
If they get it for the right price, they would then be able to sell it back to me with a little add on in price for their time/trouble etc.
Either way, it seems a Dealer must Technically take it back before you can buy it back for less than the Buy Out in the Contract,.. so there are ways to get it for less