The cheapest way would be to do it yourself.

Silkscreens cost about $20, the photo emulsion and sensitizer another $20, a squeegee costs another $20. The special 'Jeffrey' hinges you need are about $30. You can build your own silkscreen machine for just one colour, or a shirt press for four colours for under $100. Four colours (White and/or black, Red, Yellow, Blue) are all you need to make all the secondary colours you need (Orange, green, purple, etc.).

You can expose the screens by using the outdoor sunlight, or an ordinary lamp. You can heat-set the inks with an iron or an ordinary dryer. You can buy the water-based inks at any art supply store.

Then, by providing the expensive part...the labour...yourself, you really save money!

You are also in complete control of the quality of the shirts. If you want something done right (and done cheap), do it yourself!

For the same cost as an initial 'run' of printed T-shirts, you now have all the equipment you need to make more T-shirts, forever.

Also, you don't really need a hundred screens. You can use just one screen (or five screens) and fill them with hundreds of designs by using them over and over. Just wash out the photo emulsion when you are done with it, 'reclaiming' it.