let's say a developer has a large game for next-gen systems. Would the cost of doing multiple DVD's for one system vs. being able to fit the game on one blue ray disc for another system be an important consideration? Is dvd manufacturing so cheap now that its not really an issue?

An important consideration for what? Whether or not to develop for a certain platform?

"Please insert disc 3 and press any key to continue" isn't a reasonable approach for games any more, so it's not a question of one Blu-Ray vs. several DVDs. It is, rather, a question of many many gigabytes of data versus only a few gigabytes of data. And the primary limiting factor there is transfer rate.

Disk production is just a cost of doing business and it is very small compared to the cost of development or marketing. One or two disks doesn't make any real difference.

Multiple DVD's are probably cheaper than a single Blue-Ray disc to produce. I would not even use price as a deciding factor here.

As google is your friend it looks like a BD is about $1.3 versus $.5 for a dvd so BD is going to be cheaper if you need more than 2 dvds. The manual will cost more than the disk.