I am a Linux expert and I program for open-source development projects.
Are you aware that not everybody uses Microsoft Windows? Most of the programs and applications developed for marketers only address that platform.
A myth I often hear about open-source is that it is always free, so you cannot make any money with it. Mozilla (Firefox, Thunderbird and so on) is open-source software and they make upwards of 50,000,000 a year. The joy of open-source is that others will develop features they want in your software without requiring payments.
Just because the software is open-source does not mean you cannot charge for that software. Providing software, and documentation costs time, which equates to costing money, so nobody complains about you charging for your product, or for consulting around your software.
Consider developing iPhone and Droid versions of your software apps….
Costs to you are variable, depending on whether you want to open-source the software, the amount of rewriting we would have to do and the platform you want to develop for. Figure out how much a 15% increase in your target market could improve your income in a year.. Take about 1/2 of that and call that your development budget. Figure less if you really want it to take a year to complete the project. lol
We can port your product to Java, or a cloud-based platform to make it auto-magically usable for all platforms. That opens the other 15% of the market who never even bother to use your great tools, or quickly ask for a refund because it doesn’t run properly on their platform of choice.