Hi. i cant tell you what to do but only advise through my own experience. I was offered to send mikey for scans etc and never took the vet up on the offer.only reason being was obv partly the cost and the fact the vet said that the scan wud really just confirm it but there was not a lot that could be done after that. i was told just to keep an eye on him make sure he's ok when running around wild and for a few weeks i was over paranoid trying to get him to calm down etc etc but after the first few weeks I realised that, no it wasn;t effecting mikey's quality of life and I was lucky that the murmur did go away. My vet reassured me though that this is very common and I'm sorry that your vet has put it to you in a way like it is life threatening. Of course i am speaking from my own experience and don't want to play down your wee one's murmur but hopefully you'll be quite surprised at the outcome. do keep us updated on what you decide to do.
xx