Sorry bro, I can't give you a definite price as the rocks come as a package together with the tank setup... I requested it to be that way as I do not want to limit to the rocks by amount (kg)... I told Steve, lump in a figure for my rocks... just make sure they are sufficiently filled with quality rocks... dun care how heavy they are in the end.
You guys can do similar arrangement also... LR is just a small part (in terms of $$) compared to the entire setup... do negotiate as most LFS will tend not to make much out of it... considering the fact that they can earn much more on equipments