Belarusian President Alexander Lukashenko earlier said the price would drop by 60-67%, and there were media reports of the deal being linked to Minsk's possible recognition of the disputed Georgian republics of South Ossetia and Abkhazia.
"The price is adjusted depending on how the prices for oil products on world exchanges take shape," spokesman Sergei Kupriyanov said.
"It is not just important to set a price, but it is also important to receive the money, so we make certain compromises," he said.
He added that prices would also decrease for other importers of Russian gas, particularly the Baltic States and Moldova.
An agreement on the principles of gas deliveries and prices was reached by the Russian and Belarusian presidents on December 22, although a price was not announced.