I have maintained my first US card for about three or four years now, paying about $3 USD monthly. The phone number I bought for $20 USD on Numberbarn and then transferred to the card. The card itself cost me around 120 RMB. Currently, many new/unopened cards have been locked, while older accounts are doing better—you can request a phone number change annually, but the number I purchased definitely won't change.

For my second UK card, I've had it for about one or two years as well. This card number is randomly assigned. The card isn't expensive, roughly 20 RMB, and I topped it up with £10, which can last 20 years. So basically there's almost no card maintenance cost involved. You can also directly buy a finished physical SIM card for 120 RMB, and the number can be changed twice. I previously registered a Telegram account that got banned, so I just switched to a different number.

Maintaining some overseas phone cards eliminates any hassle with overseas registration. I think it's well worth it.