128gb Android phones can be had for sub-$400 these days, but options for 128gb tablets are far more limited and on average cost twice as much. I don't believe it's because of screen size, which on its own, should only cost fractions more. If anything, phones are technically more difficult to manufacture, with the need to miniaturize components and pack in LTE receivers.
Expandable storage isn't a cure-all because android often makes you jump thru hoops if you want to bypass internal storage when installing apps.
Do you have a specific phone and tablet in mind? It’s very hard to determine why without factoring in the details. Also, phones should be cheaper because it is produced at a much larger scale than tablets. Tablets sales have been stalling, if not declining, for some years now. Hence, manufacturers seem to pay less attention to it.