The glycerin cost $7.00 at Natural grocers for 16oz and the gelatin was about $2.00 for 4 pkg. I would say you can get the 9 x 13 for about $13.00 even if you add more gelatin. My 8 x 10 GA plate cost $28.00 from Amazon with prime shipping. (which means I didn't pay shipping)
I would imagine that the GA plate will last much longer, so if you really plan to do a lot of printing you might need to invest the $13.00 several different times over what would be the life of the GA plate. Of course you could spend less than $13.00 and make a smaller plate.
The 8 x 10 size is a nice size. I did make a 8 x 10 and a 3 x 5 and a 3" round out of my 9 x 13 plate. Being able to remelt it and make it the size you need for the project you are working on is a plus too in my book.