I think IPO will be closer to the $60b you mention. Plus Yahoo, will most likely only sell half of their shares at IPO. so if $60 billion, and Yahoo owns 20% (with dilution), then Yahoo will get $6 Billion at IPO, after tax $4 Billion, and they will wait til value of Alibaba is $100 billion to sell the other 10%. So another $10 billion.
Why would YHOO want to sell any more of Alibaba stock? Alibaba is a growing company and will be worth more and more over time. And, who knows; Alibaba may start paying a dividend. You really never want to sell a cash cow.