Equity in a new startup

I'm a iPhone/iPad developer by trade. A startup that I knew of was creating a product and service and they asked me to build an iPhone app in May '09. We discussed the requirements, and I signed an SOW that was for a defined set of requirements. I however, went above and beyond those requirements and built something that met those requirements but created a much better UI than the SOW asked for; thinking there would be some compensation for that at a later point. I just completed the app, and the hours i spent were 2X the hours stated in the SOW. I know, my fault but I believe in it. I enjoyed it. That SOW included a convertible note with a value of $7500. My max conversion rate is $4/share as stated in the note. The company's valuation per share was $0.74 in Oct 2010 based on a $150K investment; making the company worth $650K. There were 1M authorized shares, 911,250 outstanding, and 88,750 authorized but not issued.

Here are the options I've been given: 1) Convert my note at $2/share, create a second note with a value, conversion rate, and deliverables defined. 2) Convert my note at $2/share, and create a second "contract" that involves profit sharing from app downloads or products sold via the app. I feel that $2/share is very high. Is it too high? Whats reasonable? What can I do from here? Any advice is much appreciated.