The old Core 2 Duo is slightly weaker than current 3rd generation Ivy Bridge CPUs. To give you an idea, the 1st gen Core i3/i5/i7 is on average 10% better performing than the C2D CPUs. 2nd gen Sandy Bridge CPUs are about 12% more powerful than the 1st gen. Lastly, Ivy Bridge is about 6% more powerful than Sandy Bridge. That basically means given the same clock speed an Ivy Bridge CPU is 30.6% more powerful than the C2D CPU. That means your CPU is about as fast as an Ivy Bridge Core i3 CPU @ 1.9GHz.
The NVidia GTX 260M is no longer the performance card it used to be. It is basically between the performance of a GT 635m and a GT 640m; closer to the GT 635m though. Compared to AMD graphic chips it is basically between a Radeon HD 7690M XT and HD 7730M; closer to the HD 7690m XT. Click following link for more info.http://www.notebookcheck.net/NVIDIA-GeForce-GTX-260M.14559.0.html
Based on what is available on Newegg, the closest match I could come up with for a current laptop is the 15.6" ASUS N56DP-DH11 Notebook AMD A-Series A10-4600M(2.30GHz) which is selling for $850.http://www.newegg.com/Product/Product.aspx?Item=N82E16834230595
It has a quad core A10 APU which I would say is comparable to the Core 2 Duo @ 2.5GHz when it does not overclock itself; I think AMD calls it Turbo Core. Using only two cores, the A10 would be a little more powerful on average than the C2D if it just maintains it's standard speed; going Turbo Core the A10 is faster. This is especailly true for games or programs that can make use of 4 cores. The Radeon HD 7730m is definitely a little more powerful than the GTX 260m. The laptop has a 1920x1080 resolution screen which I assume your Alienware has as well.
The fact that your Alienware is old and no longer has a warranty are the two main reasons why it's value has dropped. Add in the fact that the battery is no longer functioning means that the price drops even further. The Asus is not an exact match since it is generally more powerful and weighs less. But it does have a smaller screen....
I would have to say that your laptop is not worth more than $400 and that is probably optimistic since a new official Dell (Alienware) battery likely costs $100 by itself.