So, I have a player that wants to play a monk who is blind, however has blindsight out to 30ft. He's arguing that even with 30ft of blindsight, he'd be at a disadvantage with anything beyond 30ft. Thoughts?

Say no. 
This comes up kind of often, where people want to play a blind character, but without any of the drawbacks of actually being blind. 
And it really opens up potential for confusion. So many things are based on things you can see. For monks in particular, if someone more than 40’ away shoots an arrow at you, does the monk get to deflect it? If an enemy is more than 30’ away, how will the monk move to engage them? Will they somehow know exactly where the creature is?

It’s just a lot of headaches and edge cases and a steady stream of rulings about every different situation. 

In my experience, at least with published adventures, most combats start and end within 30'. Given that the Monk is pretty much a melee combatant (in 2014e anyway), then that's not really much of a disadvantage.

Blindsight is a pretty powerful boon. It lets you "see" things without the need for actual sight. This isn't an even trade. Instead, I'd give them Blindsight as a skill they learn in place of one of their magic items that they'd pick up. Dagger of Blindsight is a rare magic item, so I'd consider giving them Blindsight at around level 10ish. Why so late? Because that dagger requires attunement. Unless the Monk is willing to give up an attunement slot for it, I'd wait until later to just give it to them.

He's probably thinking about Daredevil and that's not how 5e Blindsight works. So clarify everything and make sure he understands what he's asking for. 

• The most commonly used ranged Monk Weapons (Daggers and Darts) already are at disadvantage beyond 20 feet. That 30 feet range is not a real penalty. There's a reason the Fighter's Blind Fighting style and Rogue's Blindsense are limited to 10 feet. His "blindsight" should be the same. 10 feet. You'll have to come up with some other penalties to justify 30 feet. 
• If you are going to give him the 30 feet, I would tell him he wouldn't just be at disadvantage beyond 30 feet. Everything beyond 30' will be Heavily Obscured/Invisible. that means he is not able to simply target them. He will need to use his Action to make an active Perception check to attempt to detect them and then move closer to them so they're within his blindsight's range.
• He may pick up Observant and maybe even put expertise in Perception (if you allow feats) to claim his passive perception is high enough to sense them. But I'd still make him use an active perception check to find anything if you're giving him a 30' range and he's in combat. 
• Because everything beyond the blindsight's radius is Invisible/Heavily Obscured they will have advantage to attack him unless he's dodging or using Patient Defense, then it's a flat roll. Advantage and Disadvantage don't stack so nothing else in addition to the dodging will give them disadvantage. 
• He can still Deflect Missiles but if nobody is within 30' he will not be able to make an attack with what he caught. If you're limiting him to 10' then I'd allow it with disadvantage against the creature that fired at him. 
• Blindsight does not "see" through or around cover. Something can be within the blindsight's radius and still successfully Stealth to "hide" behind cover. 
• He will automatically fail any ability check that requires sight beyond the blindsight's range, no matter how high his Perception is. 
• I would also give him disadvantage on any Perception checks while the group is travelling. 

30 feet blindsight is a non-starter. Absolutely do not allow that, it's much more powerful than you think it is.

One way you could compromise would be to allow the player to pick up the Fighting Initiate Feat as a Background feature in order to take Blind Fighting. This grants a blindsight radius of 10 feet. In exchange, they would take disadvantage on any attacks they make outside their Blindsight radius (as though subject to the Blinded condition), and they automatically fail all Perception checks that rely on sight. Maybe they get advantage on perception checks that rely on hearing, but 5e doesn't actually have any hearing rules so it's hard to say how powerful that would be.

This is a hack and may still have unforeseen consequences. Frankly, D&D 5e isn't designed for disabled characters, and anything you do that allows a disabled character to function in this system will require breaking the game somewhat. I personally think that a game that can't accomodate ability diversity is a game that deserves to be broken, but you should nonetheless know what you're getting into.

Quote from FossMaNo1 >>

So, I have a player that wants to play a monk who is blind,

 Stop right there. Tell them no.

No No No No NO.

Just No.

You're the GM, say NO.

I would disagree that "I want to be 'cosmetically' blind" is an invalid character concept, but for typical dungeon crawling purposes 30' is probably too much.