If a single mistake is made (that is, if a letter is slightly malformed), it can't be erased. Instead, the entire page is discarded.

Therefore, in order to avoid discarding large swatches of text due to a single minor error, it's written on small sheets then stiched together afterwards.

It's a full time job for a whole year by a specially trained person to make one of these. Naturally it's going to cost tens of thousands of dollars.