It is your cheapest and most efficient option, by far.
Having lived in St. Louis for a great long while, I can think only one other affordable alternative.
SFSTL is barely still in St. Louis County, and thus, it is served by Bi-State (our transportation entity).
The "Eureka Express" will get you very close to Six Flags, to one of the so called "Park-Ride" lots. But you'll have to go the extra 1-2 miles on your own. Here's a map of all the bus routes:
http://www.metrostlouis.org/MetroBus/Maps/SystemMaps/MOSystemMap123107.pdf
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