Brent Roske, an NPP running for the CA-33 congressional seat, is the second candidate to drop out, following Democrat James Graf. Even if you drop out in California, you're still on the ballot. Since Roske is unknown and didn't plan to spend money, it's difficult to gauge how much more support Roske would've generated if he'd stayed in the race.
In 2012, the California Senate race had 24 candidates, including Dianne Feinstein, who was popular and had universal name recognition. She managed to garner 49.3% of the vote. While there will be more candidates spending money here, it's unlikely any candidate will be as well liked or well-known as Feinstein was at the time. The most candidates in a congressional race was 13, in the CA-8 race. Here are the 9th through 24th finishers in the Senate race and 9th through 13th in the congressional race.
What we see here is that there's a floor for a candidate just by having his or her name on the ballot. The one candidate who got less than 0.5% was the second candidate in a minor party. Based on the 2012 voting patterns I conclude that the bottom ten candidates will get 7-14%, even if they don't spend any money. Even out of the race, Graf and Roske are likely to get 0.5%-1.0% of the vote.