The fully talented 5/5 ability provides 50% and applies to the damage bonus or component derived from critical strikes. Since critical strikes hit for 150% of a normal strike, that critical bonus or component is 50%, half (50%) of which is 25%. Therefore, taking this talent makes your fire spells crit for 175% of their normal damage.
Note that this talent has powerful synergy with Ignite. Since Ignite is not based on the critical bonus, but rather the total damage, Ignite provides an additional 40% of all Burnout's 175% critical damage. This raises total fire critical damage to 245%, compared to the untalented crit base of 150%, when both Burnout & Ignite talents are taken.
The mana cost increase can be made up for and surpassed by the mana gains of specializing into Master of Elements.