I just found out you can buy a prepaid iPhone SE 32GB for $200 from AT&T and from Verizon. Prepaid = no plan.
If you are going to use it on Wifi only (since it is a spare for us), with AT&T, there is a “plan” where you can pay 25c per minute (basically emergency connectivity only), then have it network-unlocked after 6 months. Cost after 6 months = $200 + $1 (SIM card) +$25 prepaid card (good enough for 6 months) = $226.
You can’t buy the SE in store at AT&T (the deal is too good), so I just got one at Best Buy.