Can you buy 10 dollars worth of cocaine?

i always wondered that or do they only sell eight balls ? because i remember one time in 11th grade some kid in my math class was trying to buy cocaine off this one dealer and he asked him how much do u have and he said oh i got 20 and the dealer just laughed in his face and walked away and im wondering because one of my neighbors sells it and my friend wants to try it (dont judge or give us that oh dont try drugs crap or if u got caught bla blah blah please and thank u were trying everything once just answer the question) and he has 20 but doesnt wanna buy a whole 20 just a 10 to try it but he doesnt wanna walk up to the dealer and say ay give me a dime of coke and end up looking stupid if they dont sell 10's so do they or not ?