First of all, spam wouldn't be free. I would sponsor my follows and those I reply to to get my messages but not random spammers to get messages to me. Who would pay for the spam? You say the spammer might just have more money but he'd not have to pay merely slightly more than legitimate users as most clients would filter it out and it wouldn't be the purpose of paying for 500 events to just eclipse me. Great, now they eclipsed me but I still didn't read their spam. And I paid to get those 500 events? No, I did not as they were not by my follows, so my budget is still fine and I request the next 500 or ideally the next 500 from people I actually care about and only after that, what others pay to get to me.