I doubt we need additional motivation for people who otherwise don't care about the forum enough to report spam. Just like the sig-bounty "motivation" to create shitposts this would just cause new headaches.
People only need another push =] to join the signature campaign you need to have :
- at least 25 merit
- a tier 2 reporter badge
Another option is : you will get extra bucks for the previous two!
...
Be cautious with something that can make some extra bucks for the people...
If the second reporter badge will be required for something (joining to campaign, etc.. where they talk about money in the end), people who really want to join, will do everything (legal or illegal) to achieve that 2nd badge...
The easiest way for our much-loved spammers is to create some fake accounts, create around 500 (or 1000, depends on the final decision of the limit) spam posts and report them on the spot... This will give them the second badge and will give the forum another 500 (or 1000) spam posts in various topics.... (and another 500 or 1000 post to check for the mods, who could check other 500 or 1000 real spam post instead...)