I redid my aft and side decks. I didn't have teak decking, just soft decks. I cut out the top layer of glass then removed the rotted and mushy balsa core. After that I sanded the exposed glass in the lower layer and vacuum bagged down a layer of 1/2" core-cell closed cell foam except in load bearing areas. After the resin holding the core down kicked I built up the load bearing areas with solid glass (alternate layers of roving and mat) to the thickness of the core. The next step was to put down a new top layer of glass. I vacuum bagged down three layers of 1708 biaxial stitch mat with layers of mat between and a final layer of mat on top (all done in one operation). I did the job in relatively small section and the work described above took about 3 days per section. I think the vacuum bagging is essential to get the core to adhere well to the glass. When all the glass work was done I faired out the surface with epoxy based fairing compound, sanded it fair and painted with two-part urethane. Fairing took about the same time as the structural glass work in terms of days. All together the project took about two weeks. The hardest part was fairing and painting. Materials (foam core, fiberglass, epoxy, fairing compound and paint) cost about $1,100 for my job. I think the labor was about 110 hours (I did it myself). To have a yard do it would likely have run 120 hours so say $6K to $12K for labor depending on local rates. Having the yard do the work would also include haul and launch. When I did the work I did it after my Fall haul out, so there was no extra hauling charge.
The trick is to do it in reasonable bits. The first one will take you a while, but the later sections will go more quickly. Also, if you do it in sections, you can use the boat after you get the top layer of glass on each section. In other words, you can do this in the water.
This was on my glass boat, not the woody shown in the picture.