It seems that there have been improvements in preparing klaf, so that it is not coated (see how the klaf on many of the old seforim are shiny) and yet easier to write on. This brings downs the weight significantly.
I doubt that the seforim are cheaper to wrote now, nor that klaf is even cheaper. Processing costs have gone up and many people request avodas yad klaf which is more expensive.
Truth is, it is easier to see what is being leined in the typical seforim of today, they very large ones tend to have the top rows farther away.
The sefore are not so small that people in the back cannot see the sefer, either way you have top be pretty close to see the actual words.
Seats in the back cost more?
5 Columns? I though you are not really supposed to open more than three?
The cost of a sefer varies greatly, depends on who is writing it, where it is written, if it totally commissioned, what type of klaf.
I am guessing a sefer now starts at about $30,000 and can go over 100,000.
You can get some of the smaller ones for less (like the ones used on the minyan buses) , but the ksav is not really very nice.
I know that someone by my shul recently bought a very beautiful sefer, and I am sure it was quite expensive, and it was shorter than the typical seforim. It really is exceptionally nice.