250 amp breakers from most manufacturers are likely same frame size as 400 amp.
The bus in the panel it may be used in as a main is likely same as a panel with a 400 amp main breaker.
The panel with breaker is likely same price as a 400 but I suppose could be difference in price just because there is probably higher production of 400 amp breakers than there probably is for 250. Length of supply conductors could make a significant difference in cost if 250 amps is all that is required.