Things aren't great for Saudi Arabia. Yes, their production costs are very limited. On the other hand, OPEC has been losing market share consistently over time. Shale and improved technology has brought a number of new producers online. OPEC flooded the market and drove prices down to try to bankrupt those producers. It worked somewhat, but it also inflicted pain on the OPEC side. Now, they've released the choke hold. Members of OPEC nations themselves have even said that $55-$60 is a sweet spot for them. Too much higher and there'll be more competition from the US and Canada shale producers among others.
ARAMCO could have been IPO'd at any time right? So it begs the question, why now. The Saudis understand the predicament they're in, and if you can get more revenue coming in, why not take it? If all was well in Kingdom I don't think you'd see this move. As for why anyone would buy the IPO, I don't know the details of the offering but I imagine some are looking at it more as a value play than a growth play unlike many IPOs.