Lowering springs are usually in the region on 100-150 i think, for just springs, then not that much to get them fitted, or if you have the tools cheaper still.
Im not ment to have played with the barchetta or it would invalidate the warranty but ive done a nice wee bit and its been back in a few times and hey have said nout so it just depends on the day you go and who you get too, plus if you dealer is a tosser it may not even be worth taking it back to them.
OZ