Cat treated for hyperthroid STILL has tachycardia

Hi, my cat is being treated for hyperthyroidism and the tests indicate that he is responding well, but the tachycardia still persists (250/min).

My vet wants him to have a echocadiogram and says he will probably need to go on some tablets regardless of the outcome.

The problem I have is that we are on one income and the cat is 18 so I am wondering if it is worth having the echo ($350 in NZ) if he is going to go on tablets anyway.

Any ideas, thoughts and info would be appreciated.

Cheers
Smeggles
Down Under in New Zealand