OK, so some energy is required to pump the gas, this should be fairly easy to calculate. Perhaps the Eu would be happy to pay this small cost to Ukraine?
Likewise the amount of gas to pressurise the pipeline a one off cost, say it is a 1m square then x length 200km?? = 200 x 1,000 x 1 = 200,000 cu metres or at a cost price of USD 400 per 1,000 cm 200 x 400 = 80,000 USD. Probably need to allow for compression so say 30 times?? then 30 x 80,000 = 2.4M USD = peanuts for the Eu to pay Russia directly for the gas Ukraine has stolen/used.

Or have i got it all wrong??

Obviously it is no good paying the gangster regime in Ukraine because they would be back next week needing another top up.

Transit gas "problem" is obviously used as pretext. Ukraine does not like to have just one contract under control of Eu. It wants two contracts: for transit and for domestic consumption. And with prices adjusted to make gas for domestic consumption essentially free. That's how it was done for years. Russia does not want this - at least not with current Ukrainian government. Why should it subsidize hostile country?

As for "one off cost" - this story goes around and around: Ukraine just takes what it needs from the pipe - and to the hell with the consequences. So this cost will be repeated over and over. This means Eu and Ukraine will be in constant conflict over this gas. And what leverage Eu does have over Ukraine Russia does not have?