Remember that there's also potential market to consider.
As everyone notes, the cost of developing a full IFR navigator involves not just developing an approved GPS receiver with appropriate fault warning, but also a full database structure, display of all that data, and showing that there won't be problems there either. That's very expensive.
Now, if you don't pursue full TSO approval, you have limited yourself to aircraft which don't need TSO approval--that is, E-AB aircraft that intend to fly IFR. That's a very small part of the market, in the grand scheme of things. Tie it into a specific EFIS package and your market just shrank a whole lot more.
Obtaining full TSO approval opens up your product to the certified world, which is an order of magnitude (at least) greater in potential market.
What's frustrating to us homebuilders is that all we're typically looking for is the IFR GPS portion. With advanced EFIS products like Skyview and others, we already have things like ADS-B traffic and weather, terrain, large moving maps, etc. Many of us "just" want an IFR GPS without all of the fancy features (like the GPS-400W). But the certified market, still often stuck with steam gauge panels and older nav equipment, is looking for the "most bang for the buck" solution, and that market wants something more than just IFR GPS. They want the terrain and ADS-B and all the other fancy features on a single big screen because for most, that's they only way they can fit it all on a panel at anything approaching reasonable cost--and they're a far bigger market than we are. Thus, all-in-one nav/com/GPS units with lots of features and big screens are what drive the market.
TL;DR: Under current rules it's not worth developing an IFR GPS unless you can sell lots of them to the certified market.