Have been 'playing' with a couple of types of hexi blocks this week. can get 500ml to boil on one Army block, with plenty of burn time left - although not quite enough to boil another 500ml. It takes 4 round millets branded blocks to boil the same mug of water. Or you could add just a couple of the round ones to the 'unused' bit of the army one for the next brew.
Prices do vary a lot, and one thing to look for is the 'cost per brew' for example, Millets sell their own brand in boxes of 24 - but you need to use 4 of these to boil 500ml = 6 brews per pack. They also stock 'Highlander' branded nice big square ones in packs of 8, one needed per brew = 8 brews per box. Both these packs are charged at £2.99. however, the Army ones are about half the price for a pack of 8
example shown here and obviously postage cost may be needed to be taken into consideration. I intend to have a supply of both types, to use in conjunction with each other - I think its handy being able to add a couple of round ones to a part used square one to reuse it.
If it makes a difference to my findings, I have been testing them this week in my Crusader cooker and mug, which has now put both these at the top of the list for use on a day out somewhere when I just want an odd brew or two. Couldn't get on with using Hexi in a Hexi stove, or the Green Heat in the Crusader, but this combination works well for me.