I think a 20lb tank will last about 2-4 years for 65 gal.
Fire/Fire extingusher supply places have had the cheapest tanks, I got 20's for 65$USA filled. 10lbs were 45$.

BTW, you are nota limestone cutter are you?
If you ever get a chance to watch Breaking Away, that was filmed where I grew up and swam as a kid. My sister is even in it.


Regards,
Tom Barr