A 'processed 0.13u wafer' at a pure-play foundry (TSMC, SMIC, Chartered) runs "approximately" $3000 (8" diameter wafer.) Process options (polysi, e-DRAM, embedded flash, # metal-layers, etc.) affect the price +/- 50%.
From here, you can estimate per-unit/cost:
8" wafer = 200mm diameter wafer = ~31415mm2 total area
(Not all of the total-area is usable)
30000mm2 / (20 mm2) = ~1500 dice per wafer (best-case, 100% yield)
Estimated raw per-die cost = $3000 / 1500 = $2/die (100% yield)
Packaging & testing add another, I don't know, let's say $1/die?
Estimated KGD (known good die) = 1500 * 80% = 1200
Final estimate:
Estimated KGD cost = (30000mm2 / 1200) + $1 = $2.5/die
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Now then, your question revolves around "how many wafers do I need to order to qualify for $3000/wafer pricing?" That's dependent on your choice of foundry.