Slidenafil? Really? From what I read, that is what Viagra is. Why is he getting that? Perhaps for to encourage blood flow.
Can't help you with a cheaper source. If you are online, you should be able to find that yourself, if available.
As a side, how old is your dog? Sure sounds like he is consuming a lot of medication. And in my opinion, the LESS medication the better. But you may have no choice now. IMO, allowing yourself, or your pets, to get on permanent 'multiple' medications, goes nowhere but downhill.
Drugs have side effects and consequences. 'You' need to know what they are, and not just depend solely on what the vet prescribes.