Well i was gonna answer your questions, but BrentS said it all.
THe easy flo will last a long time. A LONG TIME. As long as you arent wasting it.
When i first got the stuff, i was using medicine droppers. So i wasted nothing.
Now a days i eyeball it, casue i'm used to using it. So i tend ot waste a bit here and there.
But it will last you. THe RTV is what i order the most. If your mold doenst come out right...you've used alot (depending on the size of the box)
Last year, working on Star wars and Justice league. I ordered RTV 3 times.
Keep in mind, you dont have to use these specific brands. You can get all your stuff from one place and save some shipping. I use these cause i have come to like them for various reasons. But they arnet set in stone.
As for the plaster o paris, I have to say..told ya.
The mold stuff, will give you problems... and you will end up
not liking it and it will end up sitting there.
Ask me how i knew... Been there. Done that. I've tried many store bought alternatives. The only thing that worked well, was silicone window caulk.