It should start at 10 sats with each successive post within 30 minutes of the first adds +100, then +1000, then +10,000 etc. With big warnings in red saying how much it costs and why. "you are posting too much too frequently, so this post costs 10x as much as your last one."
something like this might work well and stop a lot of the link spam on the front page... or not. they might just make different accounts and post on those to circumvent.