How to cut costs when fabricating large PCBs?
10*8 isn't really large, but you may get only 2 per panel which will impact the cost. Panel sizes will vary from fab to fab so it is worth talking to a few - and negotiating the details - as The Photon says, you may get 4 per panel and half your price that way. And the economics of setting up a fab for small jobs dictate that buying 50 boards instead of 10 can half the price again, 100 or more even lower.

In addition to The Photon's good suggestions, use the simplest process possible for the large board. Single-sided PCB may be MUCH cheaper than 2-layer, especially since there's no through-hole plating stage, omitting silk screen and solder mask may save a little more money. Some fabs may still offer phenolic material - much cheaper than FR4 fibreglass.

You might be able to use an Arduino-sized full spec PCB to hold the complicated stuff or customization, which you then plug into a much larger single-sided board which - because it omits the personalization for a specific project - you can buy in larger quantities and re-use for multiple projects.