If it's not a new ish car, best bet is to go to a breakers, or try the online parts sourcing tools to get a second hand one. Some makes/ models can run into the hundreds for light clusters.
It was a Toyota corolla 2008...not exactly sure of extent of damage the light was well and truly broke though. I am going to have charge the cyclist for this but was wondering what would be reasonable? €100??
Depending on the model, new tail lamps are €30-60 on MicksGarage, and you'll get 2nd hand for less.
What's reasonable is what it costs you to put it right. It's easy enough replace these clusters yourself, but if you're not handy, ask your mechanic what they'll charge for it.
Was thinking on this, why not just get a quote from a body shop or garage. Or a couple and then you won't be out of pocket and the job will be done right