I'm not trying to test anyone's patience. Remember, I didn't grow up with many people wearing suits, or advising me on anything like that. I may have book smarts, but I'm sure as hell not very street smart, so I have absolutely no clue.
To me, it seems like a gigantic money sink which isn't necessary. I may be saying the equivalent of "Lets go eat some babies!" to people in the professional world. I don't know these things. Call me ignorant.
Verbosity, you seem really sweet. I don't want to be mean, but I do think you're being very persistent about your position without understanding the real differences. This is what I recommend...
Go to Men's Wearhouse or some other men's store and act like you're going to buy a really nice suit. Tell the salesman you're looking for a suit that is fitting for a lawyer at a big-name law firm.
Once you're in the suit, wear it for a bit. Look at yourself in the mirror. Notice how it fits your neck and shoulders perfectly with no gap at the back of the neck. See how it makes you look. Feel the fabric. Notice the quality of the details like button holes and pockets. When you've had enough, thank the guy and tell him you'll think about it.
Now, go to your thrift store and try on your $15 suit. See if you notice a difference.
After you've done this, come back here and tell us what you've decided. If you still think the suit from the thrift store is great, at least you'll know you made a fully informed decision.