Spellcasters do not get an increase to their CR for using their spells.
A spellcaster's ability to cast spells is already a part of their CR, and you don't need to factor it in separately. You don't increase a fighter's CR because she's wearing armor, so you don't increase a cleric's CR because she's using defensive spells.
On the other hand, someone might try creating a monster that wasn't actually a spellcasting cleric but rather was cleric-themed, with powers from the cleric class. In that case it might be fair to talk about CR of individual powers.
CR is not the best tool to use, especially for monsters that have daily abilities.
CR is a very, very rough approximation for monster difficulty. CR assumes that a monster's attack damage and defense ability will be constant across the whole fight, and for spellcasting classes that's not true at all. When you try to compute CR for monsters that use spells, you get weird effects like the monster changing its CR in the middle of the fight (because it uses a more or less damaging attack spell, or because its defensive spells go up or wear off), and the CR system does not handle that well.
The questions you're really trying to ask are: will this encounter be fun for my group? and will this encounter be the right difficulty level for my group?. You could answer these questions by trying to compute a CR number, but you'll get a better answer if you think about their tactics.
How do you expect this fight will go? Do you have some specific spells in mind which the clerics can use in combat? Will those spells be a minor threat to the party (like spiritual weapon) or a major threat to the party (like harm or fire storm)?
If you have a battle where three clerics all cast fire storm in the first round, that's a very different battle from one where they all cast spiritual weapon and spirit guardians and then stand around healing each other.
Are the clerics' healing spells going to be strong enough that the party just can't kill anything until all the healing spells are depleted? Are the spells going to be weak enough that the party can focus-fire and take out one guy at a time?
Answering these questions will give you a much better picture of your encounter than just computing the CR.
But there's a good argument that the sanctuary spell should be CR +0.
When a creature is under sanctuary, we can expect that around half of attacks will fail. (This is a very rough approximation. If the creature has allies, any attacks that fail might be able to target the allies instead. On the other hand, attacks from fighter-type characters are likely to fail more than 50% of the time, because those characters usually don't have good Wisdom saves. But "half" is as good a guess as any.) Cutting the number of attacks in half is like doubling the monster's hit points. On DMG p274, I see that doubling a CR1 monster's hit points increases its Defensive CR to 4 (double hit points puts it in the CR5 row, but its AC hasn't gone up, which subtracts one). However doubling a CR10 monster's hit points puts its Defensive CR at 20. That's a big increase!
On the other hand, while the sanctuary is up, a creature's damage per round is 0, so its Offensive CR is also 0. If we really wanted to compute a CR for this creature, we'd average its very high Defensive CR with 0 and get a number that wasn't very useful.
In practice, the creature is going to drop the sanctuary and attack, so we should compute its CR using its normal stats.
Consider switching from daily spells to abilities that function every round.
If you really want to compute a CR number, you need to get away from the "spells per day" mechanic. Instead, create cleric-themed monsters that can use the same spells every round forever. When you do that, your monsters will have constant defense and constant damage-per-round, and you can look up those numbers in the CR table.
As one example: consider giving your cleric monster a sanctuary power that doesn't go away when they make attacks. Then give the monster spells like spiritual weapon and spirit guardians, calculate the DPR, and look that up to calculate offensive CR.
Here's an example cleric-themed monster.
You didn't specify a level, so this guy will be CR4. I'll call him the Spirit Caller. He has sanctuary and spirit guardians up at all times. (Technically spirit guardians requires concentration, meaning it could be dispelled if he takes damage, but he can ignore that rule.) His battle plan is to take the disengage or dodge action; he moves away from the melee characters and tries to get the ranged attackers caught in the spirit guardians effect. With his bonus action he heals himself using healing word, which he has three of per encounter.
The Spirit Caller's base damage is 3d8. 3d8 damage is 13.5 average, but we'll assume he can hit two targets per round: the fighter who charges him, plus whoever he moves next to. Call it 27 damage, then, and that might still be an underestimate because he deals half damage even on a successful save. Checking the table on DMG p274, 27 DPR is the low end of CR4; we'll say that the save DC is 14 to match.
What about defenses? A normal CR4 monster has 120 hit points and AC14. (An actual level-4 cleric has much less than that.) Our monster will have 60 hit points, and we'll treat that as 120 hit points because of the sanctuary effect as described above. We'll keep the AC14, which should put this squarely in the middle of CR4 for Defensive CR.
We still need to account for healing word which adds a total of 16.5 hit points assuming the Spirit Caller survives three rounds. Rather than add those hit points to the monster's effective total, we'll just subtract 10 hit points from his maximum: he has 50 hit points (rather than the 60 we said above), and we'll still call him CR4.
The Spirit Caller is interesting tactically, especially if there are multiples of them present at once. He wants to be next to as many enemies as possible to deal lots of damage, but for many reasons he doesn't want to be next to allies. He particularly doesn't like ranged attackers, because ranged attackers can deal with the sanctuary effect better than melee attackers can. If the party has ways to control enemy movement, now's a great time to use them.
