One major cost factor is just getting the tank to you. On the west coast it's cheap (relatively speaking) to have a large tank delivered to you - glass or acrylic. But unless you can find one in a LFS, having a big tank sent to you in other parts of the country can be a MAJOR expense. And said expense is FAR larger if you buy glass.
I built a rimless glass 75g because shipping tanks of that size cost more than the tank. That said - having done that - I strongly recommend buy vs. build if you can afford the shipping. My next tank - a custom 180g acrylic - cost half of the tank cost to have it shipped to me. But had it been glass, it would have far exceeded the tank cost.
Some glass manufacturers like Glass Cages have delivery trucks that go to major US locations that may be near you - at little or no cost. But do your research on ANY manufacturer before purchasing.