This idea would be good as for players who need a border expansion can do so, without spending all of our gold. Or add another way to expand your base, such as medals or a set amount of thorium would make it easier
#Add a one time cheap base expansion deal
opal hamlet
hybrid compass
You get 1 free border expansion via medals already, this is a premium part of the game
warped musk
Also only buy expansions when border wars offer is up. Much cheaper than buying them at their base price
foggy pumice
Like @hybrid compass said, the first is resource cost only. There are some planned changes coming that may effect this but we have nothing confirmed.