You don't lose more LP for surrenders compared to simply losing.
That said, people at low elo are really bad at assessing game states and ending games, so just because you're down 10 kills doesn't necessarily mean the game is a lost cause, and Ive seen loads of games where the enemy team through their lead and we eventually won despite people trying to surrender, so it's potentially worth it to play games out.
If the game is actually lost, then they'll be able to close out within a few minutes anyway so you're not wasting much time by not surrendering, and if they don't that means theres still hope for a come back since they're being cocky and/or don't have any macro sense until you drag things out to the 50 minute mark well games are pretty much 50:50 based on who gets caught out first.