Need instant quote form for my web site

I am a proofreader, currently building a web site ([login to view URL]). It's far from finished so don't judge too harshly. I'd like to add an instant quote calculator and order form to my Rates page. Here's an example of exactly what I want: [login to view URL] (scroll down to "Get and Instant Quote and Place Your Order"). Based on a supplied word count, I'd like the table beneath to calculate different rates based on various deadlines. I can't figure out how to do this on my own, though it doesn't seem too difficult. My preference would be for someone to teach me exactly how to create this. I have a Wordpress site, and have recently purchased Gravity Forms. I've figured out how to make a simple calculation, like adding two fields, but can't seem to figure out the rest. I'd like to know how it's done so I can create similar calculators for business proofreading/personal proofreading/proofreading for non-native English writers, etc. I'd also like to be able to change the rate calculations on my own, if I change my rates in the future.