I have a few questions about Blessed Ammunition in IH( p.190).

It states that the price is 50ea (ea =?)
I would presume its 1 bullet or bolt (in addition to normal price), but it also ablies to flamer ammution, so will 50 thrones get you 1 canister or 1 round burst worth of blessed promethium?

I believe it's 50 thrones for a pistol/basic/heavy cannister of blessed promethium. Flamer fuel doesn't come in discrete units smaller than that, unlike bolt shells or stubber rounds, which have to be blessed and etched individually.

I'd consider it to be one shot - with flamers and other weapons that employ tank-like magazines, you'll either have to fight with an only partly filled tank, risk mixing the blessed fuel with the normal or try to get a whole tankful.

On the other hand side, I really can't conceive a scenario in which the price of blessed ammunition would be important... either the mission and the acolytes are deemed important enough to merit it or they aren't. Anything else has a certain... video game feel to it: "We want you to storm that castle! Should you require siege weapons to do so, we've got a nice collection for sale..."

some puritan fire brands do like to be prepared just in case they meet the worst of the emperors enemies.

and for the flavor some acolytes likes to start with one holy bullet

Considering that normal bolt ammo is 16 per shell, a single holy shell for 50 is correct. I agree with Lathi, for flamers it is the cost for one canister of blessed fuel, meaning 2-3 shots depending on whether it is a pistol or basic flamer canister.

For reference, a bolter/bolt pistol can already do quite a lot of damage, especially considering it is Tearing. Most daemons rely on their high Daemonic Toughness to avoid damage and last longer. Negating that with a holy shell (basically adding 4-6 points of damage) is huge. A flamer doesn't do nearly as much damage per shot, so gaining multiple shots keeps it more in line with costs.

It gets even worse if someone with a bolter gets a psycannon bolt shell for their bolter, which gets even more benefits.