Some design tricks to reduce costs:
1. Use a solder cup DE9 and a 2-sided board. Put "board edge" pads there, 4 on one side and 5 on the other. The 1/16" PC Board material will fit between the two rows of solder cups, and you solder straight to them. These DE9 connectors are dirt cheap, much less than the right angle solder pin style.
2. Similarly, use
this connector, again edge mounted to a 2 sided board.
3. Make the boards yourself. You don't need plated-through holes. PM me if you need to learn how to fabricate several hundred of these cheaply.
For those thinking of stress on the connectors, particularly the RJ-45, use a potting compound to reinforce the connectors.
For 100 boards:
DE-9F: 0.60 ea
RJ-45: 0.589 ea
PCB material: 1" x 1.5", 24 per 6x6 board, 0.5525/ea
Resistor: call it 0.10
Misc: 0.50 (etchant, developer, solder, potting compound, etc.)
Total: $1.80 each for 100
And you can probably cut that a little more by careful selection of suppliers.
EDIT: If that's not an RJ-45, similar ones in RJ-12 or RJ-14 (whatever it takes) can be had for a little less.