Personally, I think there should be a sloped cost, but at a much lower speed and capped out. First I think that Bunny should be reduced to 50% of what she is now in both affection and upgrade cost. If Bunny is then taken as the base value (100%), each additional girl should cost 10% more compared to Bunny up to the 20th additional girl (so 21 total including Bunny) so that it never goes above 300% of the cost of Bunny. This would make it about 1.5 million cash or 75 kobans to upgrade a girl to 5 stars at the cap and about 2 million in items, which can be obtained in a week or two if you check in every few hours. It may take a while to max every new girl out this way, but it helps in preventing money from being useless at higher levels while not making it too hard for players who just started out.