To be pedantic, 1.5e8 carries additional information about accuracy: $150,000,000 ± $5,000,000. By contrast, 1.50e8 would be ten times as accurate, and so on.
At the very least keep it to multiples of three, as in 150e6 or 150 * 10^6! Though a very simple 150 million is easy to think of in terms of erasing past donations that sounded impressive.
