I'm planning on running a Pathfinder campaign in which the PCs have very little gold. How much should I adjust the CR of encounters to compensate? (If it's important, the gp will be probably about 25%-50% normal)
You can’t. It’s just not that simple.
CR is an inaccurate, unreliable tool in the best of times. Numerous creatures have stats comparable with other creatures several CRs higher or lower, and on top of that the power of a given party of PCs is wildly variable as well.
And it was designed with wealth in mind, and changing wealth does not affect everyone equally. The most important function of wealth is magic items—that is, magic. Classes that have their own magic are inconvenienced by low wealth, but classes without their own magic will find that they just have no way to respond to a variety of challenges, because those challenges require magic.
Worse, the classes without their own magic, the ones most severely affected by this change, were already the classes that are, by a large margin, the weakest in the game. Pathfinder is dominated by magic. Classes that had to pay for the privilege were already behind, but now they can’t even pay—they just don’t have it.
The only way to play a low-wealth campaign is to very, very carefully consider the lack of access to magic for non-magical classes when considering each and every encounter. You cannot simply apply some constant factor to CRs and expect a good result: you will not get one.
Rather than change the CR I would leave some abilities out that are problematic for low wealth groups. You should reduce DR x/magic or ignore it completely. Early on this is the most common problem with low wealth games. And in the same vain be careful with monsters that apply permanent debuffs that the group needs to pay for to have them removed. Negative levels, curses and such.
What I would strongly suggest is using the "automatic bonus progression" optional rules. Those rules tell you to only give half the normal wealth to players but gives them the bonuses normally gained through "the big six" (ring of protection, cloak of resistance, amulet of natural armor, magic weapon and magic armor and forgot what the 6th is) just by levelling. By using this rule your martial characters get their most important bonuses even with no money at all and by that they can keep up better.
Experience, character level, CR, wealth and treasure are all related. While you can adjust the rate of progression, you can't really adjust the progression of just one item, like treasure, or it throws off the anticipated power for a given level of a character. It would be easier to halve the experience and treasure per encounter. This would slow the overall rate, but keep the wealth and power the same for a given character level.
The (closest thing to a) official guideline(bottom of the page, original link doesn't work any more) is that an NPC with PC wealth has its CR increased by one, so the reverse should also apply: PCs who are significantly below WBL can handle encounters as if they were one level lower.
However, I'm only mentioning this for the sake of completeness. As the other answers say, CR is an imprecise tool at the best of times and individual encounters and monsters can be affected more(or less). For example, any encounter with incorporeal undead can be extremely challenging if the characters don't have magical weapons, and can actually be impossible if the party doesn't have a cleric or other caster with right spells or abilities.
This feels a bit like a XY problem: you ask how you should adjust the encounters for a low gold setting - but why do you want to give players less gold in the first place?
In my game, a level 3 PC has acquired about 80 gold (and has spent about half of it) - far from the 3,300 guideline. This is, combined with adjusting the listed costs, aims to make the economy a bit more interesting than "I have 1000 gold pieces and a night in the inn costs 1 silver". Yet there is no reason to adjust CRs as the PC is wealthy by founding a couple of weapons and potions worth hundreds of gold.
In other words, if you want to restrict their liquid capital, you can do it without changing CRs: just provide weapons, magic items etc in the loot or by quests. This does limit their options, on the other hand, makes items more special and doesn't suffer from the "magic mall" syndrome (it is also possible to tweak the prices of common items but that (a) is more work (b) can have unrealistic results).
If, on the other hand, you want to restrict their actual wealth to limit their access to items then you basically want a low-magic setting. In that case, you could just remove the item availability directly - and look into questions about low-magic settings.