I don't think there is a backlit mechanical keyboard for less than $100. Additionally gaming mechanical keyboards generally do not entirely support full N Key Rollover. Only the W,A,S,D keys support N Key Rollover from my understanding.
The Razer BlackWidow Ultimate is probably one of least expensive backlit mechanical keyboard for $107.
http://www.amazon.com/Razer-BlackWidow-Ultimate-Mechanical-Keyboard/dp/B003ZJ5B4I/ref=sr_1_9?s=electronics&ie=UTF8&qid=1323994354&sr=1-9
The Razer BlackWidow is the same keyboard for $80, but no backlight.
http://www.amazon.com/Razer-BlackWidow-Mechanical-Gaming-Keyboard/dp/B003ZJ1VD8/ref=sr_1_14?s=electronics&ie=UTF8&qid=1323994354&sr=1-14
Mechanical keyboards tends to be expensive to begin with since each key has it's own switch rather than relying a one single rubber membrane to register key strokes.
The SteelSeries 6Gv2 Mechanical Gaming Keyboard is probably one of the best mechanical gaming keyboard with full N Key Rollover support, but it $107 and is not backlit.
http://www.amazon.com/SteelSeries-6Gv2-Mechanical-Gaming-Keyboard/dp/B0038X3ZVM/ref=pd_sim_e_1
If the G510 is a bit expensive and you don't want the LCD screen, have you considered the G110?
http://www.amazon.com/Logitech-920-002232-Gaming-Keyboard-G110/dp/B002RRLQIO/ref=sr_1_2?s=electronics&ie=UTF8&qid=1323994354&sr=1-2