Is 5G service more expensive?

I currently have a 3 line plan with 3GB of shared data for $30/month (total.)  I just got an iPhone 15 but the other 2 lines will remain incapable of 5G for the foreseeable future.  It's difficult to tell from the XM online upgrade process whether I can upgrade to 5G (knowing only my phone will be affected) without an increase in cost.  The site implies that 5G is no extra cost.  Looking for anyone to confirm that.  I don't need 5G but if it was free, I'd take it!