Case 1: I mine in secret, I get the full profit from 12 FPGA's
Case 2: I release the bitstream for coin-X publicly; I keep mining coin-X with my 12 FPGA's, plus I get 4% of the revenue of the 250 public FPGA's (0.04 * 250 = 10).  So now I essentially 'have' 12 real FPGA's + 10 'virtual' FPGA's = 22 FPGA's.  BUT, since the network hash rate doubled, the profit for each FPGA is now half of what it was when I was mining in secret.  So my 'new' 22 FPGA's are reduced to just 11 FPGA's if you compared them to when I was mining in secret.  This is less than the 12 FPGA's I started off with.

So, with a 4% fee, I actually make less by distributing the software publicly than I would by mining in secret.  The development fee would have to be more than 4% (in some cases way more) to actually make more by making software public.