It was 1 kopek for regular, 3 kopeks with syrup. Roughly equivalent to $3 and $9 in today's US dollars if calculated proportionally to median income. Prices are shown in one of the pictures, btw: http://gazirovka.15kop.ru/gallery/#10
PS. There's much more info about soda machines in Russian version, and lots of cool graphics too:
http://gazirovka.15kop.ru/machines/ (scrolls horizontally, in case you haven't noticed)