Hmmm it is a nice dress but it depends how formal the wedding is, I would usually say not formal enough for a wedding especially as mother of the groom, unless they are having quite an informal wedding (sorry)
Also if mother of the bride ends up in a much more formal “MOB” style outfit are you going to feel ok with that? You will need to be in photos together.
Could you ask the bride and/or her mum for opinions?
It doesn’t matter how much it costs but I think the style isn’t quite right
Can you keep it to wear another time anyway if you love it? I would wear that to Sunday lunch, a summer party etc.