As you can probably tell STW isn't the place for asking questions about bike weights Weightweenies will give you far more sensible answers.
The lightest Weightweenie saddle and seatpost is around 130g all in so start there and work up to something that is practical for you. My pick would be a tuned Thomson Masterpiece (~140g) and whichever of a Selle Italia SLR/ Specialized Toupe/ Bontrager XXX or similar suits you best (~140g). That should save you around 320g which in weightweenies terms is massive. Attention to detail gets a low overall weight bike and patience with picking up bargains gets it cheaply Price-wise, "don't race what you can't replace"!
Does weight matter in the grand scheme of things? Probably not too much- most of the pro- tour road bikes are around 7.5kg although it does feel damn nice riding a light bike