I buy my grain from a local brewery, it's only $35 for 55lbs of 2 row. I also get my yeast from there at $60 for 500 grams, and hops usually run me around 6 to 8 bucks a pound, so I buy them in the 11 pound box, split them up and vacuum seal them and throw them in the freezer.
This is pretty much what I do also. My chiller is quite large, it was used in a dairy, I send boiling water through at the end of the process for about 15 minutes to really sanitize things up.
You can't go by the full and refill marks.. They can go from full to almost empty real quick. Sounds like you are just fine. A 5 lb tank will last a long time.
Just take the tank to a welding place like (air gas) and exchange it for a new one. It will be the easiest thing to do. I had a tank filled with beer gas that I got from a friend, and they just gave me a tank filled with C02.