Why is 'The apple is 3 yuan' written '苹果三块' and not '苹果是三块', or are both acceptable?
'苹果三块' (apple, three yuans) --> short for: '苹果(的价格是)三块（一个/一磅)'
'苹果是三块 (Apple is three yuans) --> short: for '苹果(的价格)是三块（一个/一磅)'
Both mean 'the price of the apple is three yuans (each/ a pound)
苹果是三块 is kind of wordy for a length reduced sentence, therefore, 苹果三块 is the preferred way to shorten 苹果的价格是三块一个 or 苹果賣三块一个
No one would interpret 苹果是三块 as 苹果 can be used as a currency for three yuans