I am considering the modification of readily available valves low temp (-40C) non-cryo rated valves in a superconductor system.  I buy from a company that provides one cryo valve (-265C) at a hefty price that im pretty sure i can get around with controlled soaks and such.  Can anyone tell me what makes a cryo valves design so superior for cold than a standard severe service besides the money spent on testing the valves functionality at cryo temps.  I am not open to vendors at this time.

I've never considered cryogenic valves to be particularly expensive.  Perhaps you need to find the right suppliers, ones that mass produce valves(Butterfly Valves).  Cryogenic valves are very similar to conventional valves with the exception of materials and features such as an extended stem.  I also don't see testing as a significant adder.

Material cost shouldn't add a whole lot to the valve.  Obviously, carbon steel can't be used.  Instead, austenitic stainless and copper alloys are used for most parts, especially pressure boundries.  Other stainless steels are used, including the precipitation hardened and martensite stainless steels.  The cost of the material is generally outweighed by the cost of machining in most cases, so although material cost is probably the largest differentiator, the cost of the valve is not proportional to the cost of material and is in fact significantly less.  

I think the primary cost difference could be quantities though, which results in different manufacturing processes.  Some cryogenic valve manufacturers don't produce enough valves to make casting and forging economical, though there are plenty of those that do and it's those valve manufacturers that are going to be most competitive.  Bar stock is used by many cryogenic valve manufacturers, especially high pressure valves where quantities are limited.  So if you're looking for low cost, look for valves with cast or forged bodies made from bronze.

The other material difference regards sealing materials, but here I don't see much cost differential.  Teflon and Neoflon (formerly Kel-F) are commonly used in both cryogenic and conventional valves.  Elastomers don't stand up well in cryogenic service and are rarely used as cryogenic seals.  However, seals are a small part of the overall cost.
 
Cryogenic valves also have extended stems to allow handles or actuators to be at ambient temp.  This adds some cost too, though the addition of a few ounces of metal isn't particularly high.  

Regarding testing, I would disagree that testing is going to be a big adder.  Most of the cost of a valve will be in the manufacture unless the quantities are low.  That's a possible issue for the smaller manufacturers but there are many larger cryogenic valve manufacturers that won't have such a signficant cost adder for cryogenic testing. 

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