I think the problem is that you don't have a verifiable skill that will increase sales by X%... Because if you did, then you'd be silly for only asking for an extra $4000 based off of increasing sales by millions.
I think software engineers can come up with a few skills that makes them more valuable than other applicants by a quantifiable amount. In my case, I have extensive experience with streaming audio and video across web and mobile devices. Having built and maintained the streaming video infrastructure that powers RushLimbaugh.com, JimRome.com, CoastToCoastAM.com, BobAndTom.com as well as their corresponding mobile sites and apps while I worked at ClearChannel, I have the experience to cut months off of similar projects for my current company. That alone was worth the 37.5% increase over my last salary to them.
I think software engineers can come up with a few skills that makes them more valuable than other applicants by a quantifiable amount. In my case, I have extensive experience with streaming audio and video across web and mobile devices. Having built and maintained the streaming video infrastructure that powers RushLimbaugh.com, JimRome.com, CoastToCoastAM.com, BobAndTom.com as well as their corresponding mobile sites and apps while I worked at ClearChannel, I have the experience to cut months off of similar projects for my current company. That alone was worth the 37.5% increase over my last salary to them.