What time do you need to pick your car up? If you're looking for cost over convenience (you did mention wanting an inexpensive options), perhaps the following options might help:

If you time things right, you can get from SNA to LAX in about two hours for $16.75 using the following connections:

Irvine Shuttle Route A: SNA to Tustin Metrolink ($1)
Metrolink Orange County Line: Tustin to Los Angeles Union Station ($8.75)
LAWA FlyAway: Union Station to LAX ($7)

You can do it even cheaper in as little as three hours, 20 minutes, by substituting the FlyAway with MTA bus 42A, which would be free with your Metrolink ticket. That reduces your cost to $9.75.

And if cost is REALLY important, you can do it for $6.10:
OCTA bus 76: SNA to Newport Transit Center ($1.50)
OCTA bus 1: Newport Transit Center to Long Beach VA Hospital ($1.50)
Metro Express Line 577: Long Beach VA Hospital to Norwalk Station ($1.85)
Metro Green Line: Norwalk Station to Aviation Station ($1.25)
LAX Shuttle G: Aviation Station to LAX terminal (free)

That actually only takes 3 hours, less than the Metrolink/Route 42A option, though Metrolink will be a bit more comfortable than the multiple bus transfers.

And actually...

If you're starting at the Newport Beach Marriott Hotel & Spa (and not the Newport Beach Marriott Bayview), you're right on OCTA route 1, which can save you another $1.50 and some extra time, since you won't need to ride OCTA bus 76. With perfect timing, you can get from your hotel to LAX in as little as one hour, 36 minutes for $4.60. That sounds like a winner!

Of course, the above minimum transit times use some pretty tight connections, and if one bus or train is even a few minutes late to a connecting stop, you might find yourself having to wait a half hour or even longer for the next connection. But even assuming one 30-minute misconnect, if you're at the Hotel & Spa, I don't think you can beat that last option--a $30-40 (depending on which hotel you're at), 20-30-minute cab ride plus a $25, 1-hour-45-minute FlyAway bus ride just seems excessive in comparison to the alternative! (Heck, SuperShuttle looks good compared to that last option!)