A $500 upcharge for 631 vs. double-butted 4130 Taiwanese tubing is ridiculous.
First of all, a comparable double-butted 4130 tubeset made in Taiwan for Nova Cycle Supply, for example, costs around
$115. That includes head tube, down tube, top tube, seat tube, chainstays, and seat stays.
A 631 tubeset goes for about
$160 at retail.
Likewise, an 853 tubeset costs about
$220.
So for an additional $1000 paid to the builder you can go from a $115 tubeset to a $220 tubeset.
I understand that 853 is a little more difficult to work with given its thin gauge and hardness, but that's ridiculous.
631 doesn't even come in any particularly light gauges. A 200 gram weight reduction isn't enormous but it's hard to imagine where the builder is getting these numbers from, unless they uses some very porky touring gauge 4130. They need to be more transparent and specific about the cost of materials and labor and what is going into this quote.