Jim,
There is a big difference in this and the others like the iLevil and SkyRadar. The Vision Pro is a 978/1090ES Transceiver, so it send your position out and receives WX and traffic IN. The SkyRadar and most others are Receivers only. A receiver only wis great for Weather via ADS-B, however if you are looking to get any dependable benefit from ADS-B Traffic, the you should really look into a Transceiver like the Vision Pro or NavWorx ADS600B. We chose to offer the Vision Pro as it is hundreds less than other comparable "Tranceivers". If you are only interested in ADS-B Weather, then look at the Receivers, SkyGuardTWX makes one as well for about $675, comparable to the SkyRadar D2.
Here is a value example, the NavWorx ADS600B Transceiver retails for about $2600, the Vision-Pro offers the same features and benefits of a Transceiver for $1399 to iFly Owners. That's a $1200 savings. And, this Vision Pro is being tested to meet the 2020 ADS-B Out Mandate. I hope this helps clear up some of the price difference.