Unsure what worms it can be used for.. But metro shouldn't cost that much? I think I paid less than $1 for a tab and one tab does 40 litres so your looking at 3.5 per day so probably around $15 to treat your tank? And if your vet charges you a dispensing fee it would add $5 but that should be max I would contact another vet, especially if they want to see the fish when they probably know nothing about it..
And I don't think fish need worming regularly as they're in a fairly closed environment, maybe if your introducing new fish regularly and also feeding live food from outside you have the potential to introduce worms..
And you would like to think the petshop would help fixing the problem but who knows if they will, have you talked to them Steve?