If you buy from manufacturer directly, then here in Mysore it costs around Rs. 80 per pipe. Then add steel cost and concrete cost, I dont think it should be more than Rs. 120.
You can do alternate poles or, design your roof truss such that it does not need any support in the middle. If going for wooden roofs, then any steel truss design can be made my nailing different members. In the photo you will see we dont have middle pillar. You can save so many pillars, but will have to go for thicker poles.
We drilled holes in the pipe and inserted small steel rod in each hole. When you fill concrete, it fixes the steel rod as well. Once concrete is set, you can bend the rods to ring/hook shape and use for tying. We had it on all four sides. You can see in the pic.
Ours was green house with shade net to be pulled when required, we were using 75% shadenet. I think this depends on how much summer sun and duration of summer.
Sides were insect net fixed permanently with plastic curtains which are pulled up during day and let down during night.
One more point is we had opening on the top similar to steel greenhouses, one of the truss members were slightly longer to have small opening on the top for air movement. Air entering from sides would escape from top.
Also orientation was north-south as we had winds from east as well as west.
I am attaching some more pics and also a diagram of our roof.
Please note that this was designed by us in our farm and we planned to use lowest cost items and use innovative solutions. So the ideas i have presented may not be fancy and will involve some extra work.