// they haven't paid because Russia hiked up the price unilaterally //
That isn't true.
Gazprom (through the Russian Government) negociated a discounted rate for Ukraine, and a contact was signed in 2010. However, the Ukrainian side have consistently broken its side of the deal, and as a result, a clause in the contract that the discounted rate would discontinued has been evoked. It wasn't unilateral, it was in the conteact. The Ukrainians knew that failure to comply would result in the discounted rate being taken away, and it has.
There is a long history of disputes over the gas pipeline and prices. Ukraine consistantly steals gas intended for EU countries, and misses payments.