How much is an Iphone if I just transfer carrier?

Hi I'm kind of new to the whole deal of buying Iphones because I never did it lol.
My Ipad has one of the old AT&T Unlimited Data plans that stopped being offered but we continuously pay each month to have it. I want to get a new Iphone probably a 7 plus because I barely have storage in my regular one. Is an IPhone still going to cost $600+ if I just try to transfer my AT&T deal from the Ipad to the Iphone? I get confused with the cost because it's always under a plan. Sorry if this question is stupid lol.

I'm not exactly sure if the question. You own an ipad with an AT@T line for it? If so you could switch the line over to a phone, instead of having to open a second line. However you'll still need to buy or lease a phone

They usually have 2 options.
1. Buy the phone outright for its entire retail price they state. An iPhone 7 Plus will be around $600, you'll have to pay this all at once.
2. Lease a phone which would be signing a 2 year contract for that device on top of the contract signed for the line. This usually costs about half the proce of Buying it, but you don't own it, it's more like renting. After 2 years you can lease the phone again, or upgrade at little or no cost. Leases are monthly payments they'd add to your bill.