I live on the Peninsula in the SF Bay Area. Looking to have a sunroom/solarium constructed on existing deck - very simple with glass walls, no heating/cooling/lighting required. Couple of quotes I got start from $30k. One said $45k. I am trying to figure out why would the sunroom cost so much?

Existing deck has house's exterior walls on two sides - so I need 2 walls + ceiling. Existing floor is pressure treated wood that is supported by piers bolted to concrete underneath. My GC looked at the deck and suggested adding piers along the deck's perimeter to support the walls - so nothing to be done for flooring other than add maybe a laminate floor over the wood for aesthetics.

I think biggest cost in terms of raw material would be the tempered glass? Floor is 15'x10' and let's say, we keep the ceiling 8' high. So the longer wall will be 15'x8' = 120 sqft. The shorter will be 10'x8' = 80 sqft. And a roof, 15'x10' = 150 sqft. A total of 350 sqft of maybe 3/4" or 1" dual-pane insulated tempered glass. Looking online for prices of tempered glass, seems like I can get the glass for $3-4k. Add another $3k for framing material etc. Still, maybe $8k on the high side for the material. Add roughly the same for labor. $15k should be reasonable. Right?

Am I missing something big and obvious here?

Thanks! :)