I would say some where in the 130-200g neighborhood.But thats a really rough guess. To be honest with ball pythons size and weigh cannot define age. It all depends on your husbandry and how you are feeding the ball python,Obviously a ball python being feed bigger prey items on a frequent basis will be bigger than one being feed small prey items on less frequent.How fast the snake grows is kind of in the owners hand,it depend on how you are feeding him.But aslong as he is eating good and not showing any signs of ilness there is nothing to worry about.
I brought my digital scale from lllreptile
http://lllreptile.com/store/catalog/...compact-scale/
They have it one reptile basics even cheaper than what i paid
http://www.reptilebasics.com/my-weig...tal-scale.html
But i see cheap scales all the time in walmart and other stores similar to it,you can probably find one for about $25 if you look hard enough.