Wookie, you don't have to get all snippy about it. It's a legitimate question, especially since the OW course basically tells you nothing about the different types of visibility options. And actually, we'll only be in Cozumel for 1 day.
I'm sorry you considered my response snippy. I try not to be snippy, I try to be forceful when it comes to safety equipment. Chihuahua's are snippy. I'm more the Rottweiler type.
I pick people up on boats for a living. I see surface signaling devises that are not sufficient to the task. If I have to look for you, or if you are overdue and I don't see your signal, I get nervous. When I get nervous, I get in a bad mood. When I get in a bad mood, everyone else gets in a bad mood, ruining the trip.
Yes, there are more reasonably priced safety sausages/SMB's out there, and maybe I mis-read your post, but it seemed that there was some urgency to you acquiring an SMB. Going back and re-reading, there was no urgency, only a desire to pay less than the price quoted at the dive shop for some upcoming trip in the future sometime to Coz.
I'll sell you a top of the line 5 foot SMB for $40. I buy them in bulk, 50 at a time, and sell them to my customers who come to the boat without one, or with a small 3 footer, or with a faded one, or one with a hole in it. Mine is safety yellow on one side for flat light conditions and orange on the other for waves/whitecaps/bright sun reflecting off the water. It isn't sufficient for tech divers, but is exceptional for recreational divers diving recreational profiles. They have a brass snap so a broken cheap plastic clip won't result in the loss of your SMB. I don't make any money on the sale of the SMB, but I think that having a really effective way to let the boat know where you are is kind of important. It's certainly worth 70 or 90 bucks at your LDS.
The most effective and user friendly SMB's I've seen are the ones from DAN, Halcyon, and Dive-rite. The DAN kit retails online for $140 from Scuba.com, The Halcyon marker I like (closed bottom) retails online for $117 at Extreme Scuba, and the Dive-Rite marker retails online for $97.75 from Dive Gear Express. So, you see, I don't think the $70 or $90 your LDS wants to charge you is all that out of line at all.
For my last bit of rottweilerness. The surface marker is for your own benefit and safety in the event you need to be seen in the water. Ask the folks who have actually used and been seen and picked up by a boat, or have drifted for 12 hours, having seen the helo but not having the helo see them. You can't have a more important and expensive piece of equipment than the one that saves your life. Will you get lost? Probably not. If you do get lost or separated from your group/buddy, will the boat driver find you anyway? Probably. If you're one of the .00001% of divers that gets left behind by the boat, even for 10 minutes, or one of the .001% of divers who get away from a moored boat and the divemasters don't spot you right away, will you kick yourself in the butt for being cheap when it comes to the one thing that will help the boat crew rescue you? Only you know or care about your own safety.