The trans has no fluid in it now, changing the filter shouldn't add that much to the cost. The reality is, if the trans is OK, you won't want to dump the new fluid to change the filter and then refill it, one less thing to be concerned about If it needs to be rebuilt, you will loose the fluid anyway, the only extra cost is the filter, pan gasket, and whatever labor to change it. If it works, your good until the next scheduled fluid change.