LOL... shit, if my coke was cut with speed I'd french kiss my dealer. It's VERY rare that a dealer is gonna cut something with an expensive substance, when they can just use lidocaine, epsom salt, baby powder, shit.. whatever the fuck they want. Don't really worry too much about it being cut unless you're injecting, cuz you're nose is very strong. My nose is the second strongest part of my body!
Coke prices are extremely tricky because the less hands it travels thru will make it purer, which in turn will probably make it more expensive. Supply and demand also plays a huge roll. You'll get a better deal in the barrio than in the suburbs. Where there's more coke dealers, there's better prices. My old room mate from West New York, NJ AKA the jersey version of spanish harlem got me a half a gram (maybe a lil more) of really good shit for $30... In whiteyville, NJ I was on a coke hunt and the prices were between 60-80 a gram. The quality of the $60 gram was better than the $80. Shit, I know a raver who sells quarter gram baggies for $20. It's a fucking rip off.
But I've never gotten in the groove of doing it regularly, so I've never found myself a steady hook-up. Good luck, have fun, and don't buy more than a gram for your first time.