Is it prohibitively expensive to buy enough Bio Spira for a 75 gallon tank?

It should run between $20 and $25 bucks.

And yes I have used it - four times on my 30 gallon. Did not work. I did see some nitrifying action after each use - but it never remained stable.

I used it four times - and the bio-spira was bought from two different stores - different batches every time.

Since I've seen so many people say that it worked for them - my conclusion is to give bio-spira the benifit of the doubt and say that my lack of results must be due to mishandling by each of the lfs's I bought from.

It's bacteria - and a lot depends on if it was shipped and stored correctly. There's no way you can tell this when you buy so - you take your chances with stuff. But most of what I've seen posted is positive.

I have used bio spira 3 times with perfect results. Where you get it from makes all the difference in the world. I have gotten mine from The Fish Store. I had it mailed to me. His prices were even cheaper including shipping than if I had found it locally. His supply is replenished weekly, and he was one of the first fish stores to carry bio spira when it first came out. He also ships worldwide.