Hi Nuttybud,
If it was your wedding, would you be annoyed/upset if people didn't book into your hotel choice? Would you actually go ahead and book somewhere that was a bit pricey? Personally speaking, I wouldn't care where anyone slept as long as they joined me on having a good night out! €300 is a bit steep to ask of anyone to spend before taking dinner/drinks/spends into consideration. All in all, its going to cost over €500 probably - are you intending on spending that much on the wedding present?
If I was you, I would go ahead and book the cheaper option, if the bride is good/close friend then I'm sure she would just be happy that you were there to share the fun and not care where you stayed.
Good luck.