In September, Steve Jobs’ $5.1 billion earned him the No. 43 spot on Forbes‘ list of the 400 richest Americans. Since then, Apple’s shares have rocketed to a new all-time high, closing Wednesday at $204.92.

So Jobs was a lot richer Wednesday than he was in September — $300 million richer, at least on paper — but only 3/5 that increase is thanks to Apple (AAPL). The rest is Disney’s (DIS) doing.

[Jobs was even richer before the market gave up most of its gains in the last hour of trading. See below.]

According to the two companies’ proxy statements, the bulk of Jobs’ net worth is in the form of preferentially owned stock:

• 5.426 million shares of Apple (most of it from a 2003 grant of 10 million shares, later reduced to pay taxes)
• 138 million shares of Disney (from when Disney acquired Pixar)

In the six weeks since Forbes took its financial snapshot, Apple has gained 32.42 points (as of Wednesday’s close) while Disney inched up 87 cents. But because Jobs’ net worth is so much more closely tied to Disney than to Apple, those 87 cents carry a lot of weight in his portfolio. Bottom line: by Wednesday night, Jobs’ holdings had increased in value …

• from Apple:   $180 milion
• from Disney: $120 million
• total:              $300 million

Shortly after 2 p.m., before the market tanked, Apple was up 9.71 points to an all-time intraday high of 208.62, and Disney was up .36 points to $29.71, together adding $103.5 million to Jobs’ bottom line.

Apple closed the day at $204.92, up 6.16 points. Disney closed at $29.23, down 12 cents.

[Follow Philip Elmer-DeWitt on Twitter @philiped]