I don't have anywhere near the detailed data that Mr. Le Roux has, but I can offer a couple of examples.
I had a client—I obviously can't name them—who wanted to dig into issues around text messaging. I can't remember what the bill was, but if memory serves, it was several thousand dollars. It may have been as high as $10,000. It wasn't a massive bill, but it wasn't nothing.
In my own firm, we didn't spend a lot of money, but that's because I spent a lot of my time—several thousand dollars' worth of time—on our newsletter list. We are employing a guy right now on contract who's going through and updating the implied consents and so forth.
I'll anticipate Monsieur Le Roux's comment. He's right: it's a best practice to do that anyway, because of course databases age and you should be updating those. Some of what someone might call a compliance cost is in fact a cost of maintaining a good database, but it's not negligible.
There are companies out there offering these plug-ins at about eight bucks a user to monitor and intervene so you can't accidentally send a spam email. Eight bucks a user doesn't sound like much, but consider that your cost of Microsoft Office, depending on the package you buy, is around $12 to $25 a user. That's a significant increment on a per-user IT cost for an organization.