I have a 10 year old with a heart murmur - he developed it later in life (before I had him, he was a rescue, discarded by someone who did not want to pay for all the vet bills we are assuming) but fortunately, he does not have underlying hyperthyroidism though we watch for that constantly. Evemn that can be handled quite easily though actually. He takes meds, eats special food, has regular vet visits with reg blood work, ultrasounds et al. The ultrasound is important because as the vet above said, it shows the amount of damage - so e heart murmurs can be what are known as "innocent" and cause no problems and some more serious. There are many degrees of severity - at least in humans - I'll leave it to the vet to respond to that in cats but I am guessing it might be similar but please ask your vet about that!!! My guy has some damage but if we look after him and he takes his meds, he should do quite well and he is. So I urge you to please see a vet and if you cannot afford kit, make a payment plan. I am sure your vet would agree and help you.
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer