What is the weight saving of windowless fuselage for cargo aircraft?
Let's take a Boeing 737 as a generic example, and spitball some numbers.
I'll make the post community wiki so someone can come along and plug in more accurate numbers.

Aluminum has a density of 2.7 g/cm3
(For our purposes let's assume the skin alloy is "close enough to pure aluminum" because I'm too lazy to look up the proper alloy density).  

Plexiglass (just picking a random variety) has a density of 1.18 g/cm3.  

The aluminum skin on a 737 is about 0.038 inches (or 0.1cm) thick.
We know a window on a 737 is thicker than the skin. I couldn't find the number but let's go with 0.25" (0.6cm).

If we have a hypothetical one-foot square window (about 930 cm2), the aluminum skin occupying that area would weigh about 251 grams. The Plexiglass equivalent would weigh about 658 grams (407 grams heavier, little more than 2.5 times as heavy).

Figure there are about 70 side windows on a 737 (quick count from a photo of a Southwest plane, not including exit door windows), so that's 28,490 grams or about 64 pounds, just for the plexiglass.

In addition to the plexiglass there's also supporting structure that needs to be added around the window to hold it in place, sealants to seal it against the skin, etc. - these add additional weight. I've no idea what these components weigh, but let's assume it weighs twice as much as the plexiglass, that's about 1.5 pounds per window of additional structure, or 105 pounds, bringing the total weight savings to just under 170 pounds.



170 pounds is not a lot of weight (in passenger terms it's not even one person), but it is that much more cargo you can carry. Also when combined with the fact that you no longer have to maintain the windows (fix pressure leaks in the seals, replace cracked plexiglass, etc.) the savings can add up over the life of an airframe.



Some more spitballing here... please feel free to correct my numbers:

On the subject of the cost savings: a 737 can carry about 27000 lbs payload, and burns about 5500 lbs of fuel / hour during cruise, and about 3000 lbs for a 15 minute climb. Assuming an average flight of 4 hours you get about 25000 lbs of fuel burnt, or almost 1 per pound of weight. This means that the fuel cost of 170 pounds of weight is about 170 pounds of fuel, which today costs (at around \$4 / gallon, and 6.5 lbs/gallon) around \$100.  Note that the cost of fuel varies a lot with location, so this is very approximate. 

Now multiply this by the typical life of the aircraft - three flights per day, 7 days per week, for 30 years? That would be over \$3M of savings. And all because your parcels can't look out of the window.