well i can get all 4 king springs in low or superlow for $205 and i can get them fitted for very little... so 150 for 2 isnt a very good price in my book.. but its probably the average reccomended retail price....
good luck with the lowering
if the above fail... try going to www.antrx.com forums... theres a bit of info on r31s, if u cant find anything start a new thread and ask for peoples opinions we are more than willing to help