We were unable to get a flat-rate in either direction in October, but the cost was at or below $45. If traffic is bad, then it could be higher, though. That's for 5 of us. If you are like me and prefer not to hassle with a rental car (we don't go anywhere else and I don't want to tote along booster seats), a taxi is nice for a higher number of people.
For a small number, I like super shuttle. There is a coupon code for $3 off each way, per person, so the cost is only $7 each one way + tip to/from SNA. I just pay the tip online when I reserve the spot so it's all taken care of when I get there. It tends to take a little longer than a taxi, though, because you sometimes have to wait for the van to fill up.
ETA: forgot to say that I read recently that you have to call ahead for the flat rate, and it's only from (not to) the airport.