+1 on the 35 liters, anything larger would be too heavy for stairs. Amount you pay has a wild range unless you get in good with the Airgas people, or, know someone. I think their list price for a 35 liter is around $120. I get my 35 filled for $25 cause I had a connection.
Larger the dewar, cheaper the cost per liter due to cool down (LN2 they use to brind down the temp of your dewar. Never made sense to me that the smaller dewars would be more, but......
If you're a college student, check with the university
good luck
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