Well, I'm not big on talking my husband into something. He really hates being hounded and pestered. The technique I find works best is, I present him with th einformation, and then walk away. I give him several days to reach the right (ha ha) conclusion. This usually gets me what I want. Of course, it helps that I know him well, and know what his arguments will be, so I try to address them upfront. And waiting is hard, because I'm anxious to have him make up his mind, but it works really well for me.
In your case, you might start with pointing out how long it's been since you've gone, how prices will only rise (esp. as kids become Disney "adults"), and so forth. You might even ask him how much he thinks would be a reasonable cost for such a trip, before you present your numbers--maybe he thinks it'll cost more than it will? And I agree with a PP who said, give him all costs, as much as possible, so he doesnt' feel like he got hit with extras after he's agreed to the cost.
And on a side note--we also have a family of 6, so I know what it's like. For the past several years, our trips to Disney have involved renting a house offsite. I don't know if this is what you might be interested in, but it would probably help to at least price out this option. A house rental (we do a 4BR in Windsor Hills) is much cheaper, with much more space, than anything onsite. You can prepare simple meals and snacks to save money. Of course, you also need a car--probably a minivan--so that's an additional cost, as is parking. But, I'm not trying so much to sell you on offsite, as to have you consider it as another possible option, to price out and present to your DH. My family likes offsite so much, we sold our DVC, but every family is different.
