"This is the average price, at which [Russian energy giant] Gazprom sells gas to all its consumers from January 1, 2009," Tymoshenko said when asked why the contract stipulated a price of $450 even though some European countries were paying less for their gas.
Ukrainian officials including Tymoshenko have said they expect the price to come down later in the year.
Gazprom and Ukrainian energy company Naftogaz finally signed on Monday a contract on Russian gas supplies to Ukraine for 2009-2019. The first gas started to flow on Tuesday.
Ukraine will hold negotiations with Russia to try to agree on a discount for Russian gas in 2010, Tymoshenko said.
"We were given a discount of 20% off the formula price in 2009. The formula price will be calculated in 2010 and we will hold negotiations on a concessional price," Tymoshenko said.