In a fighter jet, what is the average weight of the ejection system?
The F-14 manual describes the ejection system as:


  The aircraft is equipped with an automatic electronically sequenced command escape system incorporating two Navy aircrew common ejection seat (SJU-17(V) 3/A (pilot) and SJU-17(V) 4/A (RIO)) rocket-assisted ejection seats.


That means the seats are an integral part of the system.

According to this page, the weight is 228 pounds (103 kg) and it has a "maximum thrust [of] 4,800 pounds for 0.25 seconds".



A cockpit seat on a Boeing 737 weighs around 40 kg if we are to trust this replica vendor, so we can say the addition of the ejection elements adds 63 kg. Of course a 737 cockpit seat is much heavier than a WWII non-existent seat (the pilots brought their own cushions). This might be useful however if you want to compare a B-52 with and without ejection seats.

Below is an image for that ejection system, and as I gather, that's a very common (average) model found on many US aircraft.

(Click for larger size.)