I'm not sure if it packs twice as big, but it will pack bigger.
Have you done the math yet? This is just for the fabric, not any of the webbing, d rings, and reinforcement material.
Sil 21.7 oz
Coated 46.5 oz
cost increase for silnylon $22.95 (but it costs $1.75 less to ship 9 yards of sil to my zip code which I guess shows the difference in packing size between the two.)
I totally understand being on a budget (I've read a couple of Dave Ramsey's books and try to follow as best as I can), but I'd personally save up for another month and go with the sil.
There are not many ways that you can lose over an ounce per dollar when it comes to backpacking.