The amplification on vector transgenic microorganisms and the subsequent purification does not seem that complicated. Also, what are the processes that biotechnological industries follow, in detail, on their production?
Making antibodies is mostly nothing you do with transgenic microorganisms. To make an antibody you first have to immunize an animal with the correct antigen. This includes have an animal facility where you can keep animals under clean conditions. If you are looking for polyclonal antibodies (meaning a whole bunch of different antibodies with different specificities) you will boost the animal before sacrificing them and isolating their blood to purify the antibodies. If you are aiming for monoclonal antibodies then the animals will be boosted as well, then sacrificed and their spleens isolated. From the spleens you make single cell solutions from which antibody producing cells are isolated and plated one cell per well of 96 or 384 well plates and cultivate them to identify cells which produce antibodies. These cells are fused with immortal cells to produce immortal hybridoma cell lines which secrete a lot of antibodies into the cell media. These antibodies still need to be characterized to make sure the really bind the antigen of choice and not something else (in the spleen a lot a these reactions take place). Just for estimations: Making one batch of a custom polyclonal antibody in rabbit with a given peptide sequence will cost you at least 5000\$ to 10.000\$ the last time we checked for it. Monoclonals are more expensive. So making an antibody is quite an investment on the first hand.
There are some pretty good articles on Wikipedia about antibody production here and here. My crude understanding is that, you will have to grow and animal and bleed or sacrifice it to extract antibodies. This process along with purification costs a bit of time + money. Additionally, antibodies for humans must be grown under more stringent conditions to avoid contaminants and reacting to human immune system.