The cost of tube is minimal.
We use thin wall stainless steel tube in our industry and currently we are buying 2.5" tube with a 1.5mm wall for £12.80 per metre. That's top 316 grade already dull polished.
For exhaust's I don't think it really makes any difference whether 316 or 304, both are austenitic (non magnetic), 316 we use for food contact surfaces or under severe corrosion conditions.
Cheap stainless exhausts are usually made from ferritic stainless steel (magnetic), which is fine, but can tarnish and dull.
If the price is right I will be interested in a X pipe.
To be honest if I had the original pipe on the bench, I could get the lads at work to make me one for beer tokens, but it would not be cost effective to mass produce with our labour rate etc for our industry.
Cheers.