A fibbit scale is $125 at Amazon.
A run of the mill glass-top scale that you (gasp) have to calculate your BMI from is about $35.
So, unless you're REALLY bad at long division and don't have a calculator, it'll take 1800 separate weighings to make up the difference (assuming it takes 2s to punch the numbers into your calculator, and that your labor rate is somewhere around $90/hr, because that makes _my_ math easy).
If you really want to get into the weeds, you could add on the price of wifi and bluetooth, but an ESP8266 that offers both of those is < $10.