Xabbarus is right take a look at the index mundi website i think you will find ukrain GDP went up by more than 6% in 2008 which means her econamy is growing so she can afford it.
http://www.indexmundi.com/g/g.aspx?c=up&v=66
There is an alternative however:
Russia wants to stay a power in the mediteranean and black seas perhapse prime minister putin and premier mevedev (spelling?) could shall we say over look payment for the gas in return for the ukraines to extend the 2017 dead line for sevastopol.
Personally i think it is just this that is forcing some of the bartering but i think we should be warey of Russia as several U.S and british analysts in the last 8 years have said and i quote norman friedman " Russia has the potential to grow back to what it once was or simply dissapear" and at the moment i think she is doing a good job of making a return.