Basically an aquarium is glass and aquarium grade silicone sealant. I'd buy glass at a glazers, and the silicone from the local fish shop.
You do not say where you are, so beyond that it is difficult to comment.
You might want to have a look at Garf but bear in mind they use inches for measurements, and often, I think he uses glass which is a bit too thin for certain tank sizes - it could be what is available in the US though, I don't know. I can get glass any thickness.
That is very dependent on location. Here, for example, the extravagant labour rates mean that having anything done costs a fortune. When I started up again here, I found that the economics were very finely balanced.
In the UK I would agree, you will probably spend more doing a homer.