Glass door pricing is mercifully linear once one fact is known: your firebox opening’s size and shape. Everything else is finish options. The tiers:
Stock doors: the standard-opening path
Masonry fireboxes built to common dimensions (most post-70s DFW construction) accept stock door sets: quality frames, tempered glass, mesh or gasket options, at the accessible end of the range. Install is straightforward masonry anchoring, a same-day visit including fitting and sealing. If your opening is a plain rectangle within standard ranges, congratulations, you’re in the cheap seats.
Custom doors: when the opening has opinions
Arched tops, extra-wide or tall openings, corner fireplaces, stone faces with irregular reveals, custom fabrication territory, measured precisely, built to order, at a genuine multiple of stock (and worth it, ill-fitting stock doors on an odd opening leak air around their perimeter, defeating the main efficiency point). Lead times run weeks; the result fits like it grew there. The full custom-vs-stock decision.