I had a hard time finding a siamese Cat5+18/2 (or anything/2) option, but I found one that was $310 for a 1000' spool. Or you can do a dual cable pull of Cat5e ($45) + 18/2 ($70/1000')
With 8 100' runs, plus a little extra, you're going to use up almost all of a 1000' spool, so that makes rough pricing easy.
Given the extra terminations for the 18/2 (labor, plus any wiring blocks) I'd say the siamese route is probably at least $150-$400 more expensive for the cable and labor ($265 or $45 for the actual cable. 16 extra times of dealing with the 18/2 terminations at 5 minutes each at $75/hr = $100. Other incidentals $50?)
A 16 port PoE switch with 8 PoE ports is $165 (Linksys LGS116P). A 16 port non-PoE switch is $85. A 5 Amp 12V supply is $37.
Your PoE powersupply is ~$43 more expensive than a 12VDC supply, that is the only component where PoE may be more expensive.
The materials and time for siamese are anywhere from $150-$400 more.
I'd say PoE is significantly cheaper, and also more elegant overall.
In addition to U2M's calculations, I'd point out that besides the extra labor for the additional terminations, siamese cable (of any combination) can be a real beyotch to work with, too: it really only bends in one plane, it doesn't form a clean bundle once you get more than a couple of them together, and some I've used makes it really hard to split the two wires apart (sort of counter-intuitive design, really).
It also needs a much larger hole - a single Cat5e UTP will slip through a 1/4" hole (though I recommend at least 3/8") whereas a siamese pair will probably need at least a 1/2" hole. This CAN be crucial in some installs, and especially in retrofits where you may need to squeeze through some tight spaces.
Finally, one big advantage to PoE: if you're using a managed switch, you can easily power-cycle the camera from anywhere.