"We would use the principle of profit equalisation" in respect of Ukraine if it joined the union, Gazprom Deputy Chief Executive Valery Golubev said.
Almost simultaneously, Ukrainian President Viktor Yanukovich made clear that his country did not want to join a Russia-led customs union, suggesting a free-trade deal with the country instead.
In his annual address to parliament, a few days before a visit to Kyiv by Russian Prime Minister Vladimir Putin, Yanukovich said Ukraine would rather stay out of the customs union.
"I think the further development of our ties with the customs union will be based on new agreements – a free-trade agreement and a possible agreement on cooperation," Yanukovich is quoted by Reuters as saying.
He said Ukraine wanted to finalise the free-trade deal with the EU this year, a move that would make joining the ex-Soviet trading bloc impossible. Putin visits Ukraine on 12 April.
Ukraine, which depends heavily on Russian gas, has long asked Moscow to cut the price but refuses to meet long-standing conditions for a discount, such as merging state energy firm Naftogaz with Gazprom.
However, a few days ago it became clear that the price of Russian gas imported by Ukraine will jump to $347 (€219) per 1,000 cubic metres (tcm) in the fourth quarter of 2011 from $264 (€185) in Q1.
"According to the price formula, we will have a price of $293 in the second quarter, $313 in the third quarter and $347 in the fourth quarter," Ukrainian Energy Minister Yuri Boiko told a TV channel.
Ukraine bought Russian gas at about $252 (€176) per tcm in Q4 last year.
Independent analysts said earlier this month that Russian gas prices were likely to rise to an average of $300 per tcm in 2011 due to a jump in oil prices.
In line with a 10-year deal between Naftogaz and Russian gas giant Gazprom, gas prices are reviewed every quarter taking into account the price of crude oil and oil products.
Compromise on FTA with EU?
In his annual state-of-the-union speech, Yanukovich expressed hope that his country would sign a free-trade agreement with the EU by the end of this year.
"We are offering compromise positions to try and iron out differences [in trade talks with the EU]," Yanukovich said.
Tariffs and quotas on Ukrainian agricultural goods were the main sticking points in obtaining a free-trade agreement which would give Ukrainian goods wide-ranging access to European markets, the president said.
Yanukovich repeated his stance that Ukraine would like to forge strategic relations with the EU and the US on the one hand and with Russia on the other, in order to maintain a "balanced" foreign policy.