It's pretty good. For a lower cost option you could always use 2x 'sleepers'; run the tubing in between; cover with sand; and top with a new wood subfloor.
Earth temps in the US range from around 42°(Colorado Mtns, and the NorthEast) to the mid 70's(Texas & Florida). So, it depends where you live.

Sand is a poor radiant method. I do not recommend it in any situation because its performance can be very difficult to predict. Not that it can't work, just that it isn't great, and isn't predictable. Also, it gives bugs a really nice home from what I'm told. Now, if it's a basement, and you can keep the bugs out, then performance may not be a real issue. But you would want a thermal break to the slab below, at least.

gypcrete is a fine method. There are many others. Roth Panel, Sandwich Method installations, Warmboard, etc.

Radiant cooling requires watching the dew point carefully. Not many players are capable of it yet and it's not a "on the shelf solution" kind of thing yet either. But some people in some regions are doing it. Radiant ceilings work better than floors though, for cooling. Also, you want to keep your cooling load low (I'm told below 12 BTUs/sq ft in cooling load is required).

Thanks! RE: SIPs coupled with radiant. Would we be enhancing VOC's and worsening indoor poor air quality, or would this mean a need for an addtional recirc system to improve interior air quality running parell if we have radiant and a very tight SIP house??

John

Radiant has nothing to do with air quality unless you condense in cooling and make mold.

However, if you are building supertight, you must account for air quality control, definitely.