I think with the overpaying we'll get something like this (obviously this isn't balanced):
Trading Village - Action - $3
+1 Card, +2 Actions
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When you buy this, you may pay $1 extra. If you do, gain a Silver as well.
Factory - Action - $4
+2 card, something something
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When you buy this, you may pay $2 extra. If you do, gain an extra copy.