Can someone please check my AT&T Next math?

i have 2 lines on phone , years able newest phone using 1 of upgrades. give wife old phone , sell oldest model. rinse , repeat. looking @ at&t next option. reluctant not able sell phone pay newest model. however, math may suggest more or less unless missing something.

old way of upgrading:
buy phone: $400
upgrade fee: $40
monthly device payment: $0
discount on plan: $0
sell old phone: $180
total: $260 (my annual cost of upgrading)

using at&t next:
buy phone: $0
upgrade fee: $0
monthly device payment: $42.50 * 12 = $509.40
discount on plan: -$20 * 12 = -$240
sell old phone: $0
total: $269.40 (my annual cost of upgrading)

difference negligible plan discount comes me downgrading unlimited data 4gb. have never used more 3gb in 2 years. unlimited text , voice , hotspot access though i've never gone on over minutes , texts.

price of sale of old phone experience selling gamestop, craigs list or gazelle in best case (50% of original subsidized value).

being lured here? missing something? unlimited data gone might holding onto sentimental reasons vs. need , use.

insights or advice appreciated. thanks!

 

math missing 1 big part.

monthly service cost.

without that, math not work.

, hundreds (thousands) of posts here go bad. not of information , costs included.


(by way, dropped unlimited , went 10gb shared plan -saved on $1,000 year)

 

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