When OnePlus released their first flagship device, it costed 299$. Now, after some years smartphones became definitely one of the most important devices in people's lives, and it's normal that prices for them are going up. Now we're very close to beat another limit, because some phones will cost even around 600$.
Pete Lau, CEO of OnePlus company confirmed that price of their new phone, OnePlus 5T will be higher than price of previous OnePlus's device - OnePlus 5, but price probably won't beat 600$ level. We don't know how big exactly will be the price, but it's rumored that OnePlus 5T in version with 64GB of internal storage will cost 519$, and version with 128GB of internal storage will cost 579$.