I think that if you look at the date of the book (a quick search on bookfind.com showed a 1959 date) you will agree with me that prices have gone up since then! I figure that things are about 8-10 times what they were then. One of the guys in the mirror grinding group (San Diego Astro Language Removed.) has a 6" from back then and he paid about $8 for it. Todays price from Willmann-Bell is $85. That's about right for value from then to now. ASM, Newport Glass and Willmann-Bell are the biggest suppliers of stuff for making mirrors. You might like to find out how much a thicker piece of glass from a glass house is going to cost you. You might be able to get something that is scrap from some other job if you are friendly with someone there. Ask around your friends and if any are in the business, you'll probably be in luck. You might also trade for it with the glass that you have. An 8" circle would be just fine. You could do initial grinding with a iron tool and finish with a ceramic tile/plaster tool (look back through my posts of earlier questions for the process) and then you'll only need one piece of glass. Good luck. The kids will appreciate the telescope if they do much of the work.