Both sizes come in a pack of ten and each pack retails for CN¥9.90 (S$2). That works out to be only S$0.20 for each battery.
It used to be sold in Mi stores in Singapore, but suddenly, I could not find it. Not sure the reason why they stopped selling it.
I bought mine from the Mi Store (小米之家) at Pearl of the Orient (东方明珠).