If you only want to measure torque for a motor when it's not in a system, then you can do it the old-fashioned way. You'll need an extra servo horn, a light stick (about 25 cm), string, a small bucket that can be tied onto the string, and some small precision weights (you can make these out of fishing weights if you're careful).
Fasten a stick to the servo horn so that the center of the stick is at the center of the horn where the motor axle will be. Then exactly 10 cm out from the center of the horn, drill a small hole to tie a string to.
Weigh the small bucket and the string together. This is your first weight. Tie the string to the bucket, then tie it to the stick.
Set the motor so the stick is horizontal. Use a servo controller to move the motor. If it moves, then you need more weight. Add one of your weights, record how much weight is in the bucket, and then try to move the motor again. I would measure the weights in grams.
One the motor won't move, then multiply the total weights by 10, and you have the stall-torque in gram-cm.
You can do the same thing with fractional oz weights and put the hole in the stick at 5 inches from the center. You'll have to multiply by five to get the result in oz-inches.
This method has its own problems, but most of the necessary items are probably in your house or easily gotten. You'll need a precision scale to measure the weights.