Large tanks with a volume of up to 240 cubic feet might cost around $20.
For tanks with a volume of 300 cubic feet or more, the cost is not much higher (around $35).
The size of your oxygen tank would depend on your welding needs.
If you anticipate that there will be a lot of welding activity, you might benefit from a larger tank.
For smaller-scale welding, a smaller tank might suffice.