In the past every now and then we were asked if we would agree for a practice intercept on a normal routine line flight. And usually we allowed that, providing they switched off their transponder so not to trigger a TCAS event which we would have to follow. That doesn't happena anymore apparently. Probably just as well, someone would of course take videos and pictures with his smartphone and plaster it all over the worlds media.
That said, before an intercept happens, it really takes some times. At least over here in Germany first ATCOs try to reach the plane on the last known frequency, on the new one (it usually happens with a frequency change), and of course on guards for quite a long time. Next they inform the armed forces, who then try to reach the plane on guard as well while the fighters are scrambled. If that doesn't work an intercept happens and that usually wakes up the crew very fast. No idea if any bill is presented to the airline, but there will be reports and reprimands for sure and if it happens too often an airline might lose its access to that airspace or end up on the EU black list.