The default Flamethrower's compression blast costs 20 ammo, the Degreaser's 25, the Backburner's 50, and the Phlogistionator's 0, as they don't have a Compression Blast.
While this may have lots of passive/team effects, this can be deadly in combat. In the 2Fort map, you can blast an enemy into the water, leaving him unable to hit you no matter what. Also, airblasting an enemy upwards will leave them disoriented, and can often give you time to chase them down, switch to secondary or melee weapon to finish them off. This is especially effective with the Degreaser, Axetinguisher, and Flare Gun combo, as you will be able to land several critical hits onto your opponent. Even if they somehow run off, such as a scout, you can still use your Flare Gun to land a critical hit on them, which does 90 damage, and unless they find a healing souce quickly, they will definitely die from afterburn and previous damage you inflicted on them.