Been watching the development of these over the last few years. Here's AMT's site where you can find information on the Titan UAV. http://www.amtjets.com
Basicly, the 88 lbs thrust engine, at full thrust, burns .31 gals per minute. Thats a sea level burn rate, as you climb, your burn rate does drop, yet the thrust almost remains the same. With 10 gals this gives you a run time of 40 minutes. Range should be somewhere around 100 miles while cruisng, or maybe alittle more doing climbs and glides. Climbs should be around 250-300 fpm. Gas substainers weigh more, as the Jet weighs 10 pounds plus fuel of 60 pds or around 75 lbs with a tank. The gas engine substainer weighs about 95 lbs, so their is a weight savings.
Also with the this Jet engine, you can do a short auto tow, maybe a climb to 2,000 agl, and still have enough fuel for 30 minutes of cruise, or 60-70 miles.
If you look at the factory cost of an added substainer gas engine, it about 20,000 euros more, which at the present exchange rate is over $27,000 USD's. Also, the gas substainers have a hard time at western altitudes to get home on. Several at Ely have used their substainers but to get back into the valley its a hold your breath experience.
The other Jet engine is 150 lbs thrust. But I have been watching this for well over 3 years and I have been lead to believe its still in development and completion could be aways off.
I think the AMT Jet Titan, at 88 lbs thrust, is a way to go if your thinking about a substainer, installation should also be pretty simple, so costs maybe something to explore.