It can start to push up that way if you count 20-ish% in Federal income taxes, 10-13% in state income taxes, 6.25% (on the first $150k for social security). 4.5% for Medicare, a 3.8% surcharge on any capital gains or non-qualified dividends. Add in a 10% sales tax and plenty of misc fees (another name for a tax) and while you might not get to 50% exactly, it's a pretty good approximation.

California marginal income tax rates for a single person on $300k are less than 10%. On the other hand, federal rates are higher than 20%. You don't pay 10% sales tax unless you're using your entire income (uh, including the stuff that was taxed away?) on items that the sales tax applies to (and you certainly aren't).

If you're a single person in California, at $300k, if you have no special tax situation, you'll pay a bit less than 40% tax. If you're married, significantly less than that.