I think that Speakout does offer an incremental data plan of sorts. True, you have to commit $10 per month for the first 100mb, whether you use them or not. But after that, I believe that 'overage' is billed by the mb at exactly the same rate ($.10/mb). So over 100 mb, you only pay for what you use. Use 140 mb and they ding you $4 more.
If your data use goes up, then Koodoo's plan is the way to go as their data charge is one third that of Speakout's.