Really hard to confirm or deny those numbers, because there are just way too many variables. That being said, I'm not sure anyone will call you crazy for the numbers you have thrown out there, so consider the ballpark at least correct.
I've created a handful of custom widgets, and the best advice I can give is to look at the code for existing OOB widgets and see how they work. You can usually get some good insight in there if you are looking to copy or model OOB functionality in any way. Of course, the first time around you will want to follow the Getting Started guide, or whatever documentation ThWx now provides on this. You can grab the widget code from the the in your Tomcat folder, under Thingworx/Common/thingworx/widgets. Your extension widgets end up somewhere else (I forget where exactly now, but its not hard to find). I often started with a modified version of a OOB widget, depending on what I was making.
Also, I found it pretty invaluable to muck around in the browser dev tools. You can see your widget code loaded in there in the CombinedExtensions.js (or.css) files. You can set breakpoints in there which helps debug stuff, but more importantly you can edit the code right in the browser tools (at least in Chrome). If you reload the page by using the Thingworx mashup toolbar's reload button (not the browser refresh button), your code changes don't get wiped out. In this way you can develop pretty well right in the browser and see instant results. Its a tad dangerous, you could lose code or crash the browser, but its invaluable as a tool to use when you are testing out how the browser reacts to certain things.
Best of luck