bwollard,
Can't say anything about those plates as I don't have one, but as far as finish quality, I can add something.
When you ask about quality, there are a couple of things to keep in mind (assuming the plate is stainless).
Material: Will most likely be 1/8 thick 304 or 316 grade stainless. 316 is usually more expensive and generally considered to be more corrosion resistant. Remember, that is a comparison of the two grades under severe conditions....would you be able to rust a 304 grade plate of stainless with diving....doubtfull. In fact halcyon advertises their plates as 304 stainless.
Finish: There are several methods to polish the imperfections from the stainless (everything from mechanical to electropolishing). A smooth finish is more expensive, and usually has less imperfections there to act as an increased surface area. Imperfections would be the place rust would form first if it was going form. (But again, we are dealing with stainless and not normal steel)
Machining: A few things I can think of. You don't want any sharp edges. That includes the webbing slots, drill holes and edges, that could cut either the webbing, your other equipment, or yourself. Check to make sure the geometry of the slots and holes and plate itself are correct, and the bolt holes are the correct size.
As far as the $74.00 is concerned, just to give you an idea. I have built backplate and accessories (working with aluminum and stainless is a hobby for me)....just to give you an idea of the cost for me (canadian funds):
316 stainless plate - $35.00
Machine shop time to do proper full length bends - $25.00
Drilling and machining slots, equipment holes, tank mounting holes, smoothing final product - Done myself (takes time, stainless is slow to work with)
So $74.00 is understandable, since working with stainless is brutal on equipment and time compared to other metals.
I haven't priced backplates from diverite or halcyon recently, so I can't really compare the prices versus the finished product. Can you speak to others who have these plates and see what they think of them. Better yet, have a look at one with another experienced backplate diver.
Just my $0.02. In the end it will be your choice, but whatever you choose, make sure it is safe in every way possible.