Big is relative and can't be measured by height. A sapling can be seven feet tall. Better indication of price would be trunk diameter--also called trunk caliper. How big is the trunk measured across one side (not circumference-although that can help as well). If the trunk is more than a couple of inches, it's a "big" or bigger, tree. If the bottom of the trunk (the most mature portion of the tree) has begun to "bark up" as in develop the rough pine like bark the cultivar is known for, it also indicates an older tree.
A big arakawa, as in three to five inches, will be pricey, like a few hundred to $500 depending on where it is and who has it...
If you're planning on air layering, you should know that portions of the trunk and branches with extensive rough bark won't reliably air layer as the bark on Arakawa can block backbudding of new branches as well as new roots...