In general, I would say that the difference between aerospace materials and industrial materials can be explained as follows:

1. Same chemical composition, and possibly the same processing method, but requirements for testing and analysis are more stringent.  This applies to Al, Mg, and steel alloys.

2. Some aerospace materials use more expensive alloying ingredients (e.g. Ag alloying additions to Al & Mg casting alloys) that make significant differences in the cost of the material.

3. Titanium and Nickel alloys are quite apart from the other two items.  Both are expensive as raw materials and as semi-finished products (castings, forgings, etc.) so this immediately sets them apart.  In order to achieve the best properties (fatigue strength, creep strength, etc.) these alloys are double or triple melted, processed under vacuum conditions, and other expensive considerations apply when fabricating, machining, cleaning, etc.  Aerospace materials specifications are written such that these processes are requirements.  With titanium being increasingly used for industrial applications, there are now industrial standards (such as new ASTM specs) that do not require triple melting, etc.

Pretty much what tvp says is correct, The grade of material is pretty much similar though i would sugges that where in commercial alloys allow a higer residual level of contamininents like fe, zn, sn pb etc, aerospace alloys and im talking about casting alloys tend to be makde from pure ingot into alloyed ingot, hence very low residual elements.

i always thought it was down to traceability and assurance.
Definately strict rulings on mechanical tests as well as certain extra tests not needed for other applications.
Eg Westland helicopters qqb671c
is basically AB2 but must be made from ingots from approved supplier. Records of tests, procedures etc are kept for longer periods of time as well.
Your basically putting peoples lives in the hands of the manufacturer

boggyB,
  You're right about the traceability and QA concerns.  In addition to the other comments made, aerospace usually requires further refining, typically through ESR or VAR processes, before ingots are rolled to wrought product.  However, aerospace manufacturers even take cleanliness requirements beyond to the initial electrode production as well as air melt products that are used in aerospace applications.  
  A leading aerospace manufacturer did an audit of our mill and their inspection included details down to scrap segregation methods in our scrap yard to prevent cross contamination of scrap.  They were also very concerned about teeming conditions, with an extensive checklist as to mold cleanliness, runner set-up, argon shrouding, etc.  Almost all of the concerns of aerospace producers have to do with microcleanliness and tramp elements, having much tighter specs than other industries, including automotive, which is traditionally a picky customer.