Let us know the price he gave you.. Probably going to be close to the 3900 after he includes labor,... which he should. I was looking at my metal today and it looks pretty good in most places. I'll look closer as I grind and sand to make sure.. there will be some 'cancer' removal but I've already done that once on a boat trailer..
A builder friend of mine once said... it's all mud and sticks, just high tech mud and sticks.
Metal is nothing more than high tech mud. Heat it up, stick it together, and grind it down. Or just buy the new frame from AS... especially if it includes axles. that alone could be 1600 for axles and brakes etc.