Like you said - coming out of retirement can be a vastly different proposition for well known, once highly-ranked players vs. unknown, lower-ranked players. As you mentioned - the well known players will likely get the assistance of wild cards from tournament directors to quickly ascend back up the rankings while the unknown player likely would not. This would affect how many rankings points each player was able to quickly regain and thus, their ranking.
No matter the popularity or previous ranking - no player gets any portion of their rankings points back. They would start back at zero unless their retirement was very short-lived and all of their previously earned rankings points had not dropped off yet (from the previous season). I assume you understand how the rolling 12-month rankings points system in professional tennis works. If not, leave a comment and I can expand on it further.
FYI - when a player announces retirement they are not automatically removed from the rankings unless they explicitly request to be removed immediately. I believe Henin was immediately removed because she requested to be. However, in Andy Roddick's case - he was still ranked inside the top 100 earlier this year (before the clay season I believe) - because some of his rankings points had not fallen off his rolling 12-month ranking yet. He is now listed as inactive on the ATP website since all his rankings points have fallen off.
Edit: You can read more about the ATP ranking system here. Basically, each player's ranking position is based on the amount of rankings points they have accumulated over the past rolling 12 months. For example, Andy Murray won the U.S. Open last year in 2012. He received 2,000 ranking points for doing so. Those 2,000 ranking points stay as a part of his total rankings points until the week before the U.S. Open in 2013. Then they will "fall off" of his total (he loses them) and he will re-accumulate whatever points he earns by his result of playing the event in 2013.
