Buches reconstituées have an approx. 2 times higher caloric value (realistically) then ordinary wood. Combined with their burning properties you could assume that you need half the volume of these compressed logs in comparison to ordinary wood. 3 times higher caloric values can be achieved under ideal circumstances i.e. an state-of-the-art, well serviced stove with afterburner system.
You should take your time to test out different producers, there is a difference in qualities of these logs due to the use of different mixes of wood. Some use too much resinous wood thus reducing the burning time of a log and producing more waste products in your system.
As for your back, length-for-length they're heavier than your normal logs ;-)
If you're going to use it as main means of heating you can also choose to use two types of logs, called "jour" (higher combustion rate, resulting in a higher daytime temperature in your rooms) and "nuit" (lower combustion rate, giving of less heat but more gently to maintain a lower temperature for a longer time in your rooms during the night. Normally you order 70% daytime logs and 30% night logs.
But you also need the appropriate place indoor to store the logs. They contain only about 7-8% of moisture when produced and act as a sponge when exposed to too much moisture, even though they are seal-wrapped. If they come into direct contact with water or rain they swell up like carton-board and are useless for your stove.
Comparing a price per stêre isn't possible, as those compressed logs don't come in stêres. But you can compare the averige price per kW of heat produced. And in that comparison these compressed logs are about twice the price of first quality, dry oak.