Bless, along with several other spells, like Commune and Dispel Good and Evil, list Holy Water as a material component. None of them list a cost for this holy water, but the items table says that a flask of holy water costs 25g. Do these spells have a cost or not?
From PHB, p. 203:
Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
In other words, if no cost is listed, then your pouch/focus should be enough. Many of the trivial components are meant for flavour/fluff/comedic effect (see: those that require guano; or the lightning spell that require fur + glass, to generate static electricity).
Also notice that Bless requires a "sprinkling of holy water," which seems to be a trivial amount, unlike like a flask.
What about the Commune spell, which explicitly lists a "vial" of holy water?
The same reasoning applies (no cost listed, so, focus/pouch takes care of it), though granted, it's where the rules start to keel under the pressure of realism. A GM would obv. not allow a player to have such a profitable activity, and he needn't prevent it by fiat: he could just reason "if that were possible, every Cleric/Pally could do it, flooding the market and erasing all profits" (or something along those lines). But again, this is just catering to realist intuitions, which can become a slippery slope very quickly.
Spell components only cost gold if they explicitly say they do.
The flask of Holy Water listed is a full flask which has a number of uses, e.g. throwing at vampires.
The Holy Water listed as a spell component would only have to be a very small amount, probably a drop or two. This is like the difference between leather armour and the 'piece of cured leather' required for Mage Armour. As long as you have your component pouch or focus, don't worry about it. But if you found yourself with a flask of Holy Water and no component pouch, you could use a little bit of Holy Water from the flask to cast Bless.
With regards to Commune, the vial of Holy Water required does not have a gold cost listed against it. Therefore, it doesn't cost anything.