Even if you sell at R500 per GB, you're not going to recoup, because in cases like yours, you'll purchase a few GBs once in a blue moon. So instead of getting a steady R400 / R500 / R600 per month, now the ISP faces a situation where he's going to get R200 / R300 / R400 MAYBE once every two or three months, IF even that much. ISPs also need to pay the infrastructure providers to "lease" the connection, so there's a good possibility of them actually paying for your line, but not getting any income what so ever from it. Let's completely forget about what it actually cost to get the fiber there in the first place...
In your case, I'd get a uncapped line, which I know cost X. X over 30 days is Y, increase the price for the accommodation by Y per night and give your clients free, uncapped internet access. You'll of course, need to make sure you're fully booked so that the cost of the connection is covered, but that's irrelevant because fiber or not you'd still want to have your place fully booked regardless.