I too would suggest the ebay way if you don't mind having the extra cloths to pack.
Last year, I was able to get the following for a grand total of $150 (all used stuff, but good contition):
Tux jacket and pants (pants hemmed to my length)
extra pair of pants (again, hemmed to my length)
full vest
bow tie
match half-vest & tie set
two white shirts
two off-white shirts
mandrin collar shirt
cuff links
shoes
collar button (matches cuff links but goes with the mandrin collar)
all shipping charges.
There were several instances where childrens tuxes were even cheaper than mens (because the demand for them is less). The only caviot is that you've got to know what size you're looking for. If you get the size wrong, you're going to be spending extra money on either sending stuff back (if the seller allows it) or buying the same thing a second time. So of the stuff is so cheap that you can get multiple sizes and still come out cheap. As an example, one seller was constantly selling three ivory colored shirts for $5. In shirt sizes, I'm sort of in between sizes and I was able to get the seller to sell me one of each size for $5.
Two of the ebay sellers I worked with and was pleased with were Mike Dante and the Tuxxman.