We stayed in YCA (Darsena Norte) in November 2019. It is advisable to e-mail ahead of time, indicating your ETA and desired length of stay. We were offered two nights free, thereafter it was about 50 USD per day for our 14m boat.
Puerto Madero has similar prices per day, but a reasonable discount per week. Note that bridge times are different during the weekend.
We also informed about staying (for free) at San Isidro, but did not get a reply to our request, other than that they would consider it.
Repair facilities, chandleries and sailmakers are almost all in the North, which you can reach by train if you are staying at YCA or Puerto Madero.
Clearing in: immigration (at the cruise terminal, avoid walking there after dark), prefectura (at PM if you are staying there / at prefectura across the street from immigration if you are at YCA – although you may want to try at PM, as it is closer) and then aduana (at Buquebus terminal). At the prefectura we were told to go to aduana first, but there they insisted we see prefectura first (so insist at the prefectura). At aduana you should get clearance for the same amount of time as your personal visa. Insist if they give you less time. Also, upon checking out aduana told us to clear out with them as well, even though we were going to stay in Argentina. The next day, the head of aduana confirmed that we didn’t have to clear out the vessel. Our friend Omar (wetdoc) was very helpful in getting all things organised (and so much more).