Absolute best bang for your buck?
Veloflex master (or corsa) 20mm tyres direct from PBK and either maxxis flyweight tubes or continental supersonic tubes (don't get the supersonic tyres....they only last a few K's so it's actually $$ prohibitive!). The tyres aren't the 170g they claim...but they are typically still 20-30g lighter than a 4000s. each. The tubes may be as much as 40-50g lighter than a grotty cheapo tube each. Up to 150g depending on the combo you are running now and the veloflex can't be beaten for ride and grip. Downside? (there always is one) is that they are not very puncture resistant. They roll fast and grip SOOOOO WELLL!!!!
Check your rim tape. If it's heavier stuff....get a light one or consider velo plugz. (save 10-20g)
alu chainring bolts (save about 15g over steel and not overly pricey)
Build a light expander or score a tune gum gum and top cap.(save about 30g)
Get smart with a dremel. NEVER dremel a frame or fork but you can start on your bar plugs. 7g?....see if you can get them under 3g.....it's free if you have a dremel.
Alu waterbottle cage mounting bolts (4g if you have steel ones but they only cost a few $$)
How much does your spanky looking carbon bottle cage REALLY weigh?X-cross cages are about 11g and cost about $35-$40 (save up to 40g per pair but not super cheap)
When replacing cable outers, consider iLinks. This is only economical if you were doing a replacement anyway.
KCNC brakes are reasonable but definitely NOT for bigger guys on the hills and I personally don't think they work well with SRAM levers. Save about 100g
Keep your eye open for a low ks second hand SRAM red powerdome cassette or DA cassette. They last a decent length of time and will take 100g off compared to a budget casette (DA shifts better, even on SRAM systems!!).
When you upgrade/replace your pedals shop carefully and get the BEST you can.
A second hand SLR carbon Ti railed variant can be picked up cheaply second hand (save up to 100g).
Look at your handlebar tape weights carefully when you next replace it (save up to 25g).
Above all else (and I cannot emphasise this enough!) chug 1 less beer and eat 1 less slice of pizza and knock off 5kg from the ENGINE of your bike....not only will the whole set up be significantly lighter but it's cardio-fuelling system will be WAY more durable, your bike will last longer and you will actually SAVE some of the $$ required to do some of the mods listed above (save 5000g, gain power and save dollars).
You all knew that anyway, right?