Are you asking about broken glass, broken twinwall, broken frame, or what?

sides are glass in redwood, which is mostly ok. top is plastic which has slid/fallen off and the framing doesn't look great.
looking for an estimate for twinwall roof, perhaps glass. floor demension, iirc, is 20x20.

You might be interested in the calculations concerning a Harbor Freight greenhouse, with polycarbonate panels. The replacement cost of panels is close to the total cost of the entire HFGH greenhouse. If you need several panels, it may be less expensive to just buy a new 8 x 6 HFGH and use the panels supplied.

Here is a link that might be useful: HFGH panel cost calculations

From what you describe, it doesn't sound too bad as a do-it-yourself project. However, I am presuming from your question that you're not very handy, and in the Bay Area this could be pricey. I suggest you utilize twinwall as it will be much less expensive than glass...which would have to be tempered for an overhead installation. You might want to contact a couple of window installers and see whether any of them are willing to do this type of repair. The better ones are able to do repairs and have some carpentry skills.

Try these:
suppliers on right column

Here is a link that might be useful: greenhouse repair