I've discovered somewhat of an edgecase bug in the gformCalculateProductPrice() function. In my form - I'm using the jQuery Uniform plugin (www.uniformjs.com) to make form elements "prettier". I have a product field with some options and when I leave the quantity field I get a "element.html() is undefined" error on line 420 of gravityforms.js.
I've traced the issue back to lines 296 and lines 310 of the gformCalculateProductPrice() function. The code uses jQuery.next() to get the input's label element but when using Uniform this no longer works because Uniform wraps the input element in a span tag.
This issue can easily be rectified by replacing jQuery.next() with jQuery('label[for="' + jQuery(this).attr('id') + '"]');
