I made this in rails over a couple of weekends after learning how to code late last year. The inspiration came from wanting to buy an iPhone 5 from ebay but not

I made this in rails over a couple of weekends after learning how to code late last year. The inspiration came from wanting to buy an iPhone 5 from ebay but not knowing what a fair price should be. After a writing a script to calculate this for myself, I thought it could be useful for others too and abstracted it into its current incarnation. Any feedback would be fantastic, especially on UX!