It seems like its best functionality is done better by cheaper cards
It tends to be outclassed by Altar, Rebuild, Graverobber, Butcher, Taxman, Hermit etc. in the things it can do. The combination of effects is powerful, but none of those cards are limited to that particular function either.
What Expand does uniquely, among other things, is:
Copper->$3 Card that isn't a Treasure
$5 Treasure->Province
Estate->Power $5->Province all on its own
This is all good, but is that really enough to justify it costing $7? What would make Expand overpowered at $6?
Is it just a thematic thing that it costs $3 more than Remodel?