Do you mean how much was it worth compared to other currencies , or how much was it worth in buying power for a person in the Soviet Union ?

According to the 'Travellers Soviet Union & Eastern Europe Survival Kit' - published in 1989 ( so hopefully close enough to help you ), it was £1 = 1 rouble , or $1 = 0.70 Rouble. These were the official rates.

The blackmarket rates aren't given , but the book says " The blackmarket is legendary. Every tour group has at least one member who makes it his business to deal with shady characters, buying roubles ten times cheaper than the official rate." - which should give a good idea of what a tourist could get if they were willing to dabble in black market currency exchange.

As for buying power, it's hard to get a good idea, due to the way their economy worked , and due to the wide difference between official and blackmarket exahcnge rates. The same guide book says that the Russian equivalent of a hot dog from a street stand was 20 Kopeks , a local beer was 80 kopeks, 5-10 roubles for a meal in a cafe. Vodka was hard to get hold of due to recent dry laws, but a blackmarket bottle could be got for about 20 Roubles , or 7 Roubles in a 'hard currency' shop, which Soviet citizens weren't allowed to visit.