Housing "Plot Class"

[Xystic]

However, she'd like to determine exactly how much gil she needs to save to get the plot she wants

I would recommend her saving up to that 7.5 million difference at minimum. The prices of lots has been directly dependent on overall gil on each individual server. Its more than likely that from 2.38 to 2.4, a lot more gil will have been generated possibly changing the prices. So its pretty hard to determine exactly how much she needs to save. Its best to just keep making as much gil as you can till 2.4.