Getting new iphone 6's... looking at the NEXT plans. Are my calcs correct?

I think your math is wrong.

The 30gb plan is currently starting at $160 for two phones. The finance payments on the phones, plus a $15 access fee for your mother in-law's phones, would be ~$85 and this would bring your monthly bill to $245 total. 

$245 x 24 months = $5880 total

If you were to renew your contract and upgrade to these phones then your bill would be $166 a month, as you said, and you would have to pay for the upgrade cost of your phones.

$166 x 24 months = $3984

$3984 + $800 upgrade cost = $4784 total

According to these numbers, it is still cheaper to keep your current plan and to upgrade your new phone. You are quite lucky to still have the old unlimited data plan. 

There is one thing I'm unsure about though and that is that you said you get a 19% discount on your bill through work. If that 19% discount could be applied to the Next program then it would actually be cheaper than your current plan. 

$5880 at a 19% discount comes out to $4762. It's only a little bit cheaper, but nonetheless.

Can you clarify more on how your discount works, so I can get you better numbers?

The access fee for all 3 of my phones is shown on the website is $175 /month. ($130+15+15+15). Ok, it looks like I double-dipped the $15 access fee. The 6+ (128gb) is $39.59 /month and the 6 (64gb) is $31.25 /month... which = $70.84...

Phones =  $70.84

Plan =  $175 -(19% discount of $24.70) = $150.30

Taxes, Fees, Etc = ~$21.70

So... $70.84 + $150.30 + $21.70 = $242.84 x (24) = $5,828.16.

Staying with current 2yr contract = $166 x (24) = $3984.00 + $500 + $300 = $4784.00.

Still feels like I'm missing something... The ATT rep yesterday said I would definately save $$$ by going the NEXT route. I can't see it.

How long would you keep the phones?  Savings comes after the phones are paid off.  What if you looked at 4 or 5 years?  I kept my first GSM phone that long.  It literally fell apart several times a day.  I got pretty good at putting it back together.  But I was still paying the contract rate as if I'd upgraded!

TS

Do not do the NEXT plan.  They will tell you whatever you want to hear and your bill will not be anywhere close to what you are originally quoted.  I can assure you of this as I was just stupid enough to do this in May 2014.  Biggest mistake I ever made, well no biggest mistake I ever made was ever using AT&T.

The NEXT plan can save you money, but only if you are on the Mobile Shared Data plan of 10GB or greater. When you factor in the out of contract discount of $25 per month for 2 years over the subsidized contract, that totals a $600 savings. Then you have to factor in the upfront  cost of the subsidized phone plus the $40 upgrade fee, which ends up costing more than the retail price of the phone, so in the long run, you will save money with NEXT over the subsidized pricing.

Well,  since you still have your older Family Talk Nation plan, and you don't use many minutes a month, it might be the cheapest option for you right now to just keep it.

If you have the Family Talk 700 plan, and if you have 2 smartphones only, it is only about $4.25/month difference between the Family Talk (with $60 for data and $30 for unlimited SMS) and the Mobile Share 10GB plan with 2 iPhones on NEXT, when you compute the cost of the phones and the activation fees, etc.  I know, because I have already made that switch.  You can choose to bump the data up to 15GB (still doubled today) for $30 more, which is still cheaper than the $15/GB "overage fee" (if you actually use that much data).  If you don't use that much data, then paying for more doesn't really make much sense.

SInceyou have the lower Family Talk plan, and a non-smartphone on your account, it will probably be a bit cheaper for you to keep your Family Talk plan and just pay the cost for the iPhones (at $499 for your 128GB iPhone 6 Plus).  Your numbers sounds much worse because you omitted a few factors like the discount on the Mobile Share plan, and you were comparing to 15GB of data when you currently are paying for 6-8GB of data (5GB LTE on your phone plus 3GB for your wife), and you missed the activation fees, and you also may have over estimated your "taxes & fees" because our bill is closer to $10-15 for 2 phones.

One other thing to consider,  there is a chance that AT&T may finally cancel the unlimited data plans which would force you to have to switch to a metered data plan too.