DS40 and DS80 are all right (and the mitsubishi cp3800 and 70) aka citizen cx-w i believe. Some of the more reliable dye-subs out ther imho...i'd avoid copal (or the fuji rebrands thereof). Cost per print is high, though. Dye sub is good for small retail stores with un-trained ops, and good for event photography (portable). If you are looking for lower cost per page you could do WAY better with inkjet, though (think outside the box, continuous ink system and you can find 4x6 media that is only a few cents a page). If I was going to do a TON of 4x6 prints, i'd use a nice, fast, inkjet with a continuous ink system. Then find a good place to get 4x6 media from. Your cost per 4x6 is crazy, I can get 13x19 glossy sheets for .50 cents a sheet, 4x6 sheets can be found for just a few cents each. Even just going to Costco and grabbing the kirkland signature (it's quite nice paper, too) 4x6 pack would be substantial savings for you. Last time I picked some up it was approx $20.00 for 300 sheets. Was under 10 cents a sheet.
Dye-sub print quality is also poor compared to a good inkjet, or even a fairly cheap inkjet with 6 inks. Check out the black levels on those dye-sub prints, lol. Watch out for banding, too. And if you ever run like a thousand 5x7s through it and then switch back to 4x6, enjoy the hilarity of what happens there (I don't want to ruin the surprise, but I promise you there will be a warning on that in the ds40 manual, it's in the mitsu cp70 manual for sure, just checked!). Dye-sub: it's for event photographers, you can do better. :P