If you look through shower pics here on houzz you will find showers that do not include glass walls...just a door like your non- upgrade. A friend has a builder grade shower like you describe. Nothing 'wrong' with it, but it does feel dark and a bit cave like.
$2300 does not strike me as an overly high cost for the upgrade to the half glass walls, but you may be at the point in the budget where you can't keep adding more costs.
Bottom line is go with with what you are comfortable with. The proposed charge is not excessive, but keep in mind if you skip the upgrade, and change your mind later it will cost you much more than $2300 to undo it and change it.

The full wall shower option would be fine and might not be worth the cost of doing the half glass.

Kivi, thanks for your comment. I am concerned about it feeling cavelike. Also, I haven't had much luck finding photos on houzz that have full walls instead of glass. I think it would give me a better perspective. Will do another search.

try putting "enclosed shower" into the search field. That may narrow down the results for you. Could you convince your builder to put in a "window" into the wall that faces the tub. Something like that should be less than the full half glass walls. There are some examples like that here on houzz also.









You get the idea...

Lighting and built-in storage (niches) will be your best friend if you go with full walls. As long as you have good lighting within the shower stall, it shouldn't feel like a cave, and having niches to store items will reduce the cluttered look of an enclosed shower. Good luck!