the 350 isn't factoring in the costs of calls/data etc.

it depends on how long you want to keep the phone for. I bought my phone in 2017 ago for £400 and I pay £8 a month to a carrier to get an allowance of data + calls every month (not a contract, I'm free to cancel any time) so it's worked out at about £18 a month if you amortise the initial outlay.

obviously contracts are more expensive, and can sometimes, albeit rarely, be cheaper, you just need to do the maths.

the bigger advantage to buying a phone outright is you're free to switch network/carrier as the phones are unlocked, at least in the UK anyway.