My son does both. I've noticed that while he can take or leave the surfing he never wants to miss time in the dojang. I told him if there was anything that could have drawn me in other than surfing it would have been martial arts. For me my family was too poor, and now I can only afford to send one of us otherwise I would be in there with him.
Have trained MMA for 5 years, much of that time in dedicated fashion. Have competed a few of those years as well. MA is very parallel with surfing on several levels, mentally and technically. It's helped me a ton in surfing to have MMA in my background. The only way it held me back a bit is that surfing is flat-footed while your demise on the mat or in the cage comes when your heels touch the canvas. You guys told me at the beginning when I wanted to grasp technical understanding to "just go and surf". Doesn't work like that for the mental/technical athlete I am. If a BJJ newbie was told "just get on the mat and roll", they'd blow their load getting gassed by going too fast and not breathing, not to mention getting tapped out in 8 seconds after putting themselves in poor position. 60-90% of MMA is done at slow speed, depending on which discipline you're talking about. We don't have that luxury to slow things down on the water and that is why I believe surfing is the most challenging athletic endeavor I've ever been in. Thank God for the Carver!!!!!!!!
Cost was an issue for us as well. My son wanted to continue BJJ, but it was so expensive (one year contracts) at the place he was going; although it was a pretty good school. But his interest would continually fluctuate. We tried finding less expensive schools, but they weren't very good. Some were black belt and/or tournament trophy mills. There was no art in their martial arts. Community rec centers may offer less expensive classes in various martial arts; but classes are often only held only once a week for an hour, filled with a lot of little kids (remember Kramer in that Seinfeld episode?), or have various safety rules limiting sparring or the types of moves that can be practiced.