Logitech G110 - Backlight and 12 "G Keys", never looked up price.
Logitech G15 - Backlight, 6 "G Keys" on newer and 18 on older style, and aux display. You should be able to find it on sale for $50-70 with a full price of $100.
Logitech G19 - Backlight, 12 "G Keys" and a full color aux display that can play video (YouTube, ect), and full powered USB ports (G15 is passive USB ports). Don't know about sale prices but full price is $200.
I have personally owned a older model G15 for a few years now and it works great. Also have the newer model of the G15 and chances are unless you troll Ebay for the older one it is the one you will get. But on my newer main system it has proven to be a good keyboard.