The basic problem with cow/pig ivermectin (which seems to be what's generally used as an OTC alternative) is that it's tremendously concentrated, relative to the dose that even a large dog needs. You can dilute it, using instructions that are pretty readily found via google (of course, dilution is just ... math).
If your dog is or may be collie or part collie (or similar -- australian shepherd, e.g.), then they have a genetic variant (or can) that makes them very sensitive (like, deadly sensitive) to overdosing on ivermectin. For other breeds, this is not as risky, and indeed there are conditions (scabies) that are treated with much higher doses of oral ivermectin than is used for heartworm prevention.
I use cow ivermectin in non-diluted form (but tiny amounts) for my ~75 lb. dog. I clip the tip off a fish oil capsule and squirt it in there, then give it orally as I would a (non-tasty) pill, being careful to keep the capsule upright so the ivermectin doesn't spill out (other than into the dog...).
And yes, it's cheap as all heck. I think the requirement of heartworm testing (for a dog on preventative) + prescription product is a pretty big and, honestly, problematic (as it means fewer dogs get preventative) scam. I get that that approach (testing, etc.) is "best practice," but in the real world where we all make decisions about tradeoffs (and here I'm not really talking FIRE, more a can-I-afford-healthcare-for-myself, which is a big issue for many Americans), I don't think it's appropriate.
When I adopted my (rescue) dog, he was heartworm positive, so went through treatment and it was awful ... would not want to do that again. But when the same medication is available OTC, I'm happy giving it that way.
Many vets will not see/treat your dog (on an ongoing basis) if they don't provide an annual heartworm test + prescription (though you could get that filled elsewhere, or not, and I don't think they'd know). Mine lives in a rural area and is willing to do so. May be worth checking with yours if that's a concern.