If you're going to use a RDBMS, you will eventually have the contents of all 929 files in one database, most probably in more than table. I cannot tell you much more about the design of such a database since you don't provide enough detail about the contents of each of those files. The exact layout will be a normalized form of your 30 million rows in probably a handful of tables. The performance of modern RDBMS is good enough to handle data of that scale if (and only if) your indexes are properly set.
There is very little reason not to put that data into a RDBMS. The only reason I could think of is to eliminate the need of such a subsystem entirely, e.g. to simplify deployment of your solution. If you actually consider doing that then yes, a set of 929 files could act as a hierarchical database. The main difference to a RDBMS solution is that with such a set of flat files you can only reasonably query your data by one key - that being your zip code (or any part thereof).