Maybe a simple question maybe not...

Mig or tig welded cage?

A competent welder should be able to use either method effectively. You'll probably find MIG is much more common as it is faster and easier, particularly in tight spots.

tig costs more

From a reliability standpoint (proper) TIG welding is much more durable, and yes it is usually more expensive.

Most MIG welding will only heat the surface of the material to be welded and glob the welding wire to the surface of the tube. On the other hand a good TIG weld will fully heat the area of tube being welded to form a "puddle" to which the filler rod is added. I have several cross cut photos of 1-1/2 x .120 tube both MIG and TIG welded, the Tig welding BY FAR penetrates the tube deeper with less gas void.

just my $.02 worth.
-Nick

I live in the land of Nascar and you would think that getting a roll cage would be easy.
Well getting it is easy but i have been told for a non cromoly tube frame (6 point)mig welder that the price is $1800-$2000. And the windshield has to come out.

Is this reasonable?

That's about the going rate. Check out Hanksville Hot Rods mail order cages, sponsor link on the left

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You would still need a good welder to put everything together.