Hello JLopatin,
After several months of work on this for our trumpet models, here is what I have learned:
Sterling silver can have wall thicknesses down to .8mm, which is right in the sweet spot for typical flute construction. However, the sections of the flute will need to be under 100mm in length (3.94"-ish). For a typical length flute you will be around 80-85 cubic centimeters of material for the tube section alone (an educated guess). At $35per cc3 you will be over $3K for the materials (easily).
With the stainless steel product, the wall thickness we have been able to produce for the trumpet body are at 1.5mm minimum. Almost too thick to be useable for a horn, but it still works for the body, valves, and valve pulls. We are buying an OEM bell from Bach/Conn.
If you go the route of stainless, you will need to add outer thickness to the tube walls to accommodate the inner bore of the flute. It will be heavy, but there are theories about conservation of energy and the standing wave form, which make this heavier instrument make sense. This is why the trumpet we are building is like a Monster truck or armored tank, where all the energy going into the horn comes out the bell, without tiring the player as quickly and giving a big sound. Looks like a tank, too. But trumpet players are all greedy for volume, so we're hoping it fits into the line up for stage playing.
That said, if you go stainless for some of your items, you will want to plate it after assembly/getting it back to your site. Shapeways/ProMetals does electroless plating, which means that both the inside and outside of the tube will be plated. This will affect your fittings.
You might look at the alumide or plating on some of the stronger resin plastics.
Right now, our trumpet parts are coming in around $1700 (with the commercial bell, too) and we have threading done at a local machine shop for the valves and top/bottom caps.
I am including a little photo to show some of our first model, done with the electroless coating in matte gold. I had to dremel off the excess material on the fittings.
Hope that helps.