It CAN be cheaper... but the price seems to depend on whether or not your LS has traction control or not. This is based on the information you have given,,, and a simple search on Google done on the actual internet... and not through "internet lite" done through a cell phone.
However.... even if you can find the ABS module cheaper,,, you still have to have the knowhow to install it... and posesss the special software/computer to properly bleed the full brake system afterwards. You may not like this answer,,, but go through the dealer that quoted parts and install.
IIRC... the ABS system on the LS is only fully bleedable by a specific process throught the dealer,,, from a special computer that is the only way to open all solenoids on the ABS module to get the air out of the system.
Yes... You might be able to find the part for $200-$400 less,,, but if you walk in with your own part the dealer will most likely not warranty the work!!! Might be time to play nice with the dealer,,, and beg them for a discount.