f(1400) = 60, simply means for 1400 text messages the cost will be $60.
what might have caused the shift after 800?
some cellphone plans, include a budget mechanism such that, if you don't text message so much, you get a low price of $30, so if you text message say 300 times or 500 times or even close to 800 times, you only pay $30. Now, if you're a Rogue messenger and text more than that, well the service provided has to be increased, because the cellphone company is doing more text message transfers, and it varies from company to company, but this one in the picture above, is charging $10 more for every 200 extra text messages.
From the company's side, it can be seen as they cannot charge less than $30 for text messages in a plan due to costs they have to get it going, so that'd be a minimum to pay for having it, at the same time they provide a reasonable amount of text messages allowed for that minimum charge.