I went to the place which I normally fill my welding tanks (lowest prices) and asked them the CO2 prices. There is no discount necessary and they are open seven days a week.
To swap an aluminum bottle:
10lb bottle $20.35
15lb bottle $22.42
20lb bottle $24.50
All prices already have taxes added in so this is what you actually pay.
Laurel Bottled Gas
10002 Washington Blvd N
Laurel, MD 20723
(301) 725-2882
Sunday 10:00 am – 4:00 pm
Monday 8:00 am – 5:00 pm
Tuesday 8:00 am – 5:00 pm
Wednesday 8:00 am – 5:00 pm
Thursday 8:00 am – 5:00 pm
Friday 8:00 am – 5:00 pm
Saturday 8:00 am – 5:00 pm