If I've left pricing on kilos - or any other bulk amounts - I'd be most grateful if you could report the relevant posts so I could snip 'em down to size. That's not sarcasm that's actually the case - it's what they pay me for innit.
As Ponch says, it depends on the drug and to an extent on which mod sees the post and their own interpretations. Not to the extent Crackers suggest but there's no set amount as such. Common sense should be enough in most cases. If you could use it all in a day - or maybe a weekend - it's personal. If it's bulk amounts cos you're stocking up for a few months it may be technically personal but it's also bulk. Imagine being busted with whatever it is. Would you be utterly crapping yourself or would you be reasonably sure you'd get a slap on the wrist of some form unless you got horribly unlucky.
For me (roughly and subject to mood swings and the like) I'd say...
Weed = 1oz is about the limit
Pills = maybe 10 or so
Coke = 3.5g is about the limit
Heroin = again 3.5g max (purely cos that was how much percy I bought every Friday when my wages went in... which lasted me all of 3-4 days on a good week 8))
Speed = will stick with the 3.5g even though I know much of it is such bad quality that's barely enough to keep you awake past your normal bedtime)
Ketamine = yup, 3.5g seems reasonable and keeps it simple (before PTCH goes and digs up one of my posts about buying 1/4 litres (12.5g) it was a bit different when you could pick that up for £70)
MDMA = that magic 1/8th oz again. Pushing it a bit but again chosen cos that was my usual percy of a week a few years back.
Meph = go on - guess. That would be correct if you guessed 3.5g again.
Acid = 25 tabs. Bottles of liquid is a bit iffy even though i know some do like to stock up.
2C-x = Tricky cos dosage is small but it's much handier to buy in minor bulk cos so much cheaper. I'd stick with just talking about 2C-B pills (same limit as MDMA pills) and just not bother pricing powders cos people buying that kinda stuff know what it costs.
Random RCs - just use common sense. And to bo honest nobody really needs to know usually cos it's essentially buying from a shop - they have discounts and sales and everything dontcha know.