This podcast interview with British thru-hiker "Ben" helped clear things up for me. He says that a three-day supply of food at a typical supermarket in a trailside town cost between $30 and $40. I should note that this was in 2006, and that both the interviewee and his hiking buddy are the same height as I (so maybe our appetites match, too?). This matches what I was expecting: roughly $12 a day for food supplies using the buy-as-you-go strategy.
Ben spent $4000 total on his trip, which lasted 134 days. Presumably about $1500 was spent on food for the trail, judging by the numbers he gave. The rest was spent on hotel rooms, restaurants, and in-town feeding frenzies. Perhaps a few hundred was spent on new gear (such as replacement running shoes). Judging from the interview, I would estimate that the hikers spent 25 nights in hotels, spending an average of $80 per night in town: $40-50 per person for a room (lodging in California is really expensive) and $30-40 on food splurges. From what I've read, $20-30 is a normal sum for a big thru-hiker dinner in a trailside town.
What does this mean for me? Hopefully, I will be able to limit my expenses to $400 a month on store-bought food and $70 a month on a fresh pair of Inov-8 trail runners. I am not attracted to hotel rooms and prefer to sleep outside for free. I can maintain my personal hygiene as I go rather than letting it slide until I hit town. Avoiding restaurants may be a bit more difficult. Rather than going in for a $25 dinner, I can get a variety of appetizing foods from the supermarket and splurge on that instead.