I was able to squeeze 2.5gal of pure juice from about 50lbs of central Tejas Muscadines. With additional work I probably could have gotten that up to 3gallons of juice so the 15lb number seems 'about right' for comparison purposes if you go the acid dilution method of adding water as noted below.
The hard part with these grapes comes in when trying to get the pH/acid under control. You can either reduce the acid level via additives, or the addition of spring water to bring it in line. If you use additives, the juice volume will pretty much be what your starting volume is and go down from there due to the settling of sediment/yeast which doesn't get transferred as the process goes along. That would leave you maybe a gallon or just shy in the end with 15lbs of grapes. If you add water to get the acid under control, you'll dilute the juice flavor in the end product and it will lighten the product up yet you also can have a larger starting volume. As a reference, I used approximately 10gal of spring water for that 2.5gal noted above of juice giving me a starting volume in the 13+gal range of a very light colored beginning juice stock which was correct for acid and SG. Leaving the grape in a nylon 'must' bag will help considerably in retaining that deep color too.
Getting the SG (sugar) under control is the easy part with a hydrometer and a LOT of sugar. Like 2lbs per gallon on average.
I'm pretty new to this whole wine making thing my self, and the folks here are wonderful in helping new winemakers such as you.