Have to second the overland route. Ferries will cost and are not quick!
Dover to Dunkirk is an option to get across the channel, I recently did Harwich to Hook of Holland as thats an overnighter, not crazy expensive, and puts you nicely on your way to the likes of Czech and then Bulgaria.
We went from there to Leipzig and on to Prague, no tolls fees for motorbikes so all free! Hook of Holland to Leipzig was a comfortable if somewhat dull day on the motorways.
From there you could strike out to Bulgaria whichever you choose - its all good riding and you will see more, pay less and take less time to do it.
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