The cost to make steel is around $300 per ton. And cost to make aluminum is around $1200 per ton. Stainless steel is about $2500 per ton. If you were buying stuff like plate, beams, or pipe without including the shipping & handling cost your price would be around $500 per ton for steel and $1500 per ton for aluminum. Your total cost could be $100 to $200 per ton more than this.

For a small pipelauncher you could need less than 1000 tons of material so about 1 million dollars worth of material. And you should able to actually construct it for another million dollars. And there would other significant costs to actually get to the point of launching a rocket, but roughly speaking it could cost less than 5 miilion.

A large pipelauncher would over be 1000' in length and probably exceed 1000 tons, it could be over a mile in length and exceed 5000 tons- since deepest ocean depth is over 6 miles, one could have pipelauncher over 5 miles in length. The cost of making a 1000' long pipelauncher twice the length is at least twice the cost [material and construction] though there could efficencies which develop which could lower costs [different material and different ways of construction].

I'm going to go over a design for a small pipelauncher which 1000' long. Because of costs you probably wouldn't use Stainless steel. The question is do you use aluminum or steel. Aluminum is stronger per it's weight than steel [2.7 vs 7.8 in density] as general rule but there various grades and alloys. An aluminum alloy might seem the obvious choice- aluminum maybe 3 times the costs but less than 1/2 the weight so in volume it's only only about 150% the cost. But it should noted that no one makes cargo or other big ships out of aluminum- and you could sort of say that a pipelauncher is a kinda *specialized* ship [and should probably be built in a shipyard].

As I've said before a pipelauncher is essentially a pipe with one end capped- but to work to launch rockets which has a mass of hundreds of tons it needs to be a big pipe. And it needs a structural design other than a simple pipe- and I think a large pipe made from smaller pipes is good idea. So you could have diameter of say 50' and have 12 60" diameter pipes equally space within the circumference {157'}. The pipes take up 60' of the 157' leaving 12 sections of about 8' in length.

Ok use this aluminum:
"Alloyed with 2.7% magnesium, 0.8% manganese and 0.12% chromium; Good formability, weldability and corrosion resistance; Often used for pressure vessels; Tensile strength between 36,000 to 47,000 psi."
http://www.engineersedge.com/aluminum_plate.htm

So the 6' diameter pipes could have 1/2" thick walls and section in between can slightly curved [as though part of 50' diameter pipe] and be 1/4" plate.

So how would 1000' of this weight?
Let's just do 1' section. The circumference of 6' pipe is 18.85' times 12 equals 226.2' which 1/2" thick. Which is 9.425 cubic feet of aluminum. The section are 8.1' times 12 which 97.2' long and 1/4" thick. which is 2.01 cubic feet. Giving total for 1' section of 10.435 cubic feet of aluminum. Times 1000 gives 11,435 cubic feet.

A cubic foot of water weighs 62.42796 lbs- http://www.fourmilab.ch/hackdiet/www/subsection1_4_2_0_7.html
So, 62.42796 times 11,435 equals weight if 1 density of 71,3863.72 lb. So this times by 2.7 which give weight of the aluminum:
1,927,432 lbs. Or 963.72 tons. If this aluminum costs $1700 per ton that would be about 1.6 million just for material. With total about 3-4 million to build.

And check the displacement, roughly: A one foot section of 50' diameter pipe displaces 1964 cubic feet or 125,696 lbs of saltwater.
So, 15.3' length of pipelauncher displaces it's weight.
Which mean that about 7.5 psig would be the pressure inside if just floating the pipelaucher. A constant 15 psig of inside pressure would cause pipelauncher to accelerate at 1 gee upward. So to launch most rocket it would require less than 30 psig.

That's basic layout. You need a cap. You might make the sections 3/8" instead of 1/4". And other things, but this could be kept under 100 tons of additional weight.

5 million for launch pad is cheap. Boeing spent 25 million for their used oil platform- then spent tens or hundreds of million modifying it. Compared a launch pad on the land, 5 million is even cheaper.
If you could rent a launch pad for 5 million per use, you are getting pretty good deal. A pipelauncher can be used over and over again. Can launch from equator- or any latitude you want. And adds a hundred mph- or faster depending upon the amount gees a rocket could take- speed to rocket.