I can get you very... rough specs. The IT28F is one of the few pieces of Cat literature that I don't have in my rather large collection.
The F-to-G transition comes with a powerplant update and some more power on the front end.
Brief specs:
F: made between '93 and '96, 125 horsepower. Approx. weight of 25,200 pounds; bucket rated at 2.6 yds with a breakout force of 21,700 pounds. (Do you need dimensional specs, too?)
G: Made between '02-'04, 146 horsepower, 27,860 pounds. Same size bucket but 23,437 pounds of breakout force.
Width, ground clearance, dump clearance, and travel speeds all increased; the max reach decreased.
Let me know if you need anything else -- I may be able to dig it up for you. Edit: I don't know anything about price, unfortunately... you may want to check out Ironplanet or MachineryTrader.