If that deal goes through, it looks like Lemieux is going to be doing incredibly well for himself from what I can gather. Some quick research and math shows:
Lemieux was part of the purchasing group that bought the Penguins in 1999. At the time, the team owed him about $32m. He forgave $7m of the debt and accepted the other $25m as equity in the team.
The team was purchased in 1999 for $107m. This would put Lemieux at a 23.4% stake assuming that $107m was for a 100% of the franchise.
Assuming Lemieux still has a 23.4% stake in the franchise, his portion of the $750m sale would be $175m. Not bad for what can be thought of as "delayed payment" for his time with the team.
Franchise value growth from 1999 to the current 2015 asking price is 601%. Annualized, the growth is 13.0% per year over 16 years. For comparison, annual inflation was about 2.4%.
Forbes currently values the team at $565m, which is still 428% total growth and 9.5% annualized growth. Lemieux would have $132m in equity in the team, which is quite respectable.
Random sources and resources:
HFboards post about Lemieux's stake
Forbes' valuation of the Penguins
A June 2015 article discussing a potential sale (Note: Google cache version because the article is behind a paywall.)