No Power--How do I diagnose further?

I am trying to help my in-laws diagnose a dead Inspiron 531 desktop computer which is no longer under warranty.

The computer gets no power whatsoever.

When I plug the computer into a good power outlet...with a good cord...I get absolutely nothing. Once connected to the outlet, the ON/OFF button does nothing when pressed. There is no booting activity, no sounds, and no button colors. The green LED on the back of the desktop box does not light, even with the cord attached. Completely dead.

(Just so you know, I used the same socket and cord to successfully bring power to their accompanying Dell monitor...so I feel confident that power is coming to the computer.)

A couple of questions:

--The lack of LED light on the back of the computer seems to suggest to me that the power supply is fried. Is there any way for me to be sure, aside from bringing it into a local store and paying $, that the power supply problem is indeed the problem? Once the power supply is replaced, is it possible/likely that the motherboard is fried as well? Any way to diagnose ahead of time how much it might run me to get it back working again (assuming I do the work myself).

(Also...any suggested power supply replacements? When I look for replacement parts for the 531 from Dell, they don't show a power supply option.)

--My in-laws reported no "smoking gun" prior to this event, aside from some issue with their cable provider that temporarily messed up their phones and televisions. They do get internet through cable...is it at all possible that something through the cable modem could actually cause a problem like this?

--I'm open to opinions on whether this computer is worth saving...or whether it should be recycled or donated. I just don't want to get in a situation where I buy one part...then another...then another, etc., trying to get it to work.

Thanks for any and all help.