I've been doing stand up in LA for a little less than a year now, so this is just my experience. Most open mics in LA are paid slotted mics (online signup, $5 for 5 minutes). but there are some free ones even if they are less common. There are new mics that pop up all the time so there are as many opportunities to go get on stage as you can find. There are also a few places that operate doing open mics all day long (the hollywood comedy/4th wall) which are usually very friendly to beginners. Around the bigger clubs is where people tend to be less nice, not mean necessarily, but if you want respect from professionals you have to earn it with your talent. I've tried to introduce myself to a few people at the comedy store who were friendly to audience members but blew me off when I told them I was an open mic'er.

Open mics at the bigger clubs (Icehouse, Improv etc.) are usually free, but comics are selected by a raffle and comics get less than 5 minutes so those mics are usually for more seasoned people looking to show off their tightest material.

I've heard that it's better to move to LA once you have a few years of experience so that when you're trying to get booked, you're not trying to convince people who have seen you bomb while developing your skills. but a ton of people (myself included) didn't start doing comedy until after they moved to the city, so the experience is really whatever you want to make of it.