Then again, Clerks was famously paid for by a bunch of credit cards. I guess there's an exception to every rule.
Smith did spend around $30k of his own money on production costs. The bulk of that was for film stock. (Similarly, Rodriguez spent $8k of his own money on Mariachi.)
Now, here's the kicker: Neither one of them spent a dime on film prints. They signed deals and let the studios take care of that. This is significant because if people talked about Clerks' or El Mariachi's budget in those terms, the numbers would be in the rane of hundreds of thousands of dollars.
To put it in perspective: Would you say you "self financed" your startup if a big multinational paid for all the servers and bandwidth from the get go? It's sorta like that.
Saying it in interviews makes for a better story ("College dropout makes major movie for $8,000!") but it's not really accurate.