The cheapest ones are about $35.00 on Lightmart.com. Why are they so expensive?

They are not expensive. They just did not get that cheap as many other things.
Unlike with the things like microelectronic, where the most expensive parts are the processing and development, which can get easily diluted over high volume production but very little amount of materials.

A good quality structural thing just need its amount of material and the material for a single unit will cost the same regardless if you are making 10 or 10Meg pieces.

The cheapest ones are about $35.00 on Lightmart.com. Why are they so expensive?

Not sure of the point of this post lol. You answered your own question in your question. They're good quality. Much thicker pipe is used compared to the paper thin wallblaster arms you can buy at Home Depot.

When I need an arm for a fixture or something, I grab a steel pipe from the yard or buy some cheap steel pipe and then I bend it, and it works just fine, for less than 5 bucks or free.

$35 is actually cheap for an arm. Over here, the distributors want around $160 for a 6ft aluminum tapered elliptical arm. The same ones used on the street. I'd get one but I have no use for a arm right now.

  Lot of that  is takinb up by manufacturing cost.    Then shipping cost and store have to make make  money as well.

   Now if u want get street light arm like I did dirt cheap.  Go to a  metal recycling yard like I do and look in there extrustion scrap pile.  I got mine for like 2 buck each.   Even some of 2 inch ones are even found once an a while.

The cheapest ones are about $35.00 on Lightmart.com. Why are they so expensive?

Thirty five bucks isn't that much for a quality made item... 
At least they're not charging an arm and leg for it. 

Thirty five bucks isn't that much for a quality made item... 
At least they're not charging an arm and leg for it. 

It is if you are 12 years old and only get $2.00 a week to save for them, and you need about 7 of them!

  Lot of that  is takinb up by manufacturing cost.    Then shipping cost and store have to make make  money as well.

   Now if u want get street light arm like I did dirt cheap.  Go to a  metal recycling yard like I do and look in there extrustion scrap pile.  I got mine for like 2 buck each.   Even some of 2 inch ones are even found once an a while.

That's also a good idea!

35 bucks for an arm? No way! Arm from scrap or DIY from pipe for 2 bucks go brrrrr! 

That's also a good idea!

35 bucks for an arm? No way! Arm from scrap or DIY from pipe for 2 bucks go brrrrr! 

The thing is, you won't get quality item that isn' at or past of its end of life (corrosion proggress,...).

To be a good quality, you need good corrosion protection, applied after all the welding. So home welding from stock piping material is out of the way.
And some old arm from a ccrap yard will have all or most of its corrosion protection layers eaten out, often even including the main material, so the strength and longevity are gone.
Sometimes a cheap thin walled thing but with a fresh protection treatment will last you longer. But then will have faster EOL progress once the protection layers get consumed (there is not much to chew from the thin wall till it really collapses)...

My mantra in buying most things is to always buy top quality products. Yes you might have to save up before getting them but generally you only need to buy once rather than replacing it after a few years.   
In the end you spend more money if you buy the cheaper alternative

Yeah, personally I'd just buy the correct arm, especially if it's only $35. The amount of tools you need to properly make a arm would cost a lot more than that.

The problem here is he needs a bunch of them so the "cheap" $35 becomes a not so cheap $250 . $250 might as well be a million when you're his age. 

If the installation leaves the arms readily accessible (not high above a permanently crammed interstate), you won't need that high quality ones (when speaking of corrosion protection; to make them last 20 years on the elements without anyone touching them), if you can afford a bit more frequent maintenance (yearly paint fix job,...). Because the main cost difference is really in the corrosion protection longevity, so maintenance requirement.
In a proffessional world a requirement for annual paint fixes would cost $100's on labor (and even more when nearly inaccessible like on a highway you are not allowed to even restrict for a day without causing havoc in traffic in 30 mile radius around), so paying $50 extra to not need to touch it over the expected 10..20 year lifetime is quite a significant cost saving.
But for a hobby use, when getting there cost you just to take a ladder from the barn 30 feet away and doing the work as a part of your hobby makes the cost calculations quite different...

The problem here is he needs a bunch of them so the "cheap" $35 becomes a not so cheap $250 . $250 might as well be a million when you're his age.

You are exactly right! Thank you for clearing that up! The reason I made this thread was just to see if there might be a cheaper alternatave. And I don`t know of a single metal recyling yard that you can go to and buy stuff for cheap in the state of North Carolina.