You can exspect to pay $1.25-$1.75 for a new, standard rectangular tank.
For a custom tank you will be paying more.
For a used but clean tank you should pay $.75-$1 per gallon.
If its dirty youd pay 50c per gallon and if its cracked but usable then you negotiate.
Tis be how i work with tank purchase. OR you go to Petsmarts $1 pergallon sale they have one every few months, their due soon.