The Urban Myth That Just Won't Die. Microsoft's alleged $150 million check was in partial settlement of a lawsuit (most likely related to stolen QuickTime video acceleration code used in Video for Windows), and was just an initial payment according to Fred Anderson, Apple CFO at the time. The exact total amounts have never been publicly revealed, but could have been up to a Billion. The settlement also included cross licensing of patents (including that stolen code), and MS promise to continue Mac versions of Word and Excel for five more years.
By the way, back then Apple had never had less than $400 million in the bank, with little to no long term debt. Never near "insolvency".
Yes a lot of the 1997 Microsoft-Apple deal has become the stuff of urban legend, but this account is not quite right either. The $150M wasn't alleged, it actually happened. It also wasn't a check, it was an investment in Apple shares, a special nonvoting class that Microsoft was required to hold for at least three years. As you say, how much cash changed hands behind the scenes was never reported. At the time Apple still held over $1B in cash, but they'd lost that much in just the year and a half before, so Apple was indeed bleeding at a rate that would have been terminal at some point, not very far off. A takeover or buyout looked pretty much imminent.
When this deal happened, it was most often reported as "Bill Gates Wins Again!" Because, you know, Bill Gates always won back then. Even getting roundly booed by the crowd when his face appeared on the big screen at MacWorld was seen somehow perversely as a win for him. Not many had the insight to look beyond the surface for what really happened, which is that Steve Jobs had brilliantly engineered a deal that disengaged Apple from their death-clutch with Microsoft and did it in a way that gave Bill Gates enough cover to sign on. I bet he regretted it later.
This is probably the single-most important and least understood thing Steve Jobs did to ensure Apple's survival. If it had not happened, very likely no iMac, no iPod, no iPhone, no iPad.