I hate ModernBill. I'd seriously consider ClientExec. ModernBill seems great on the outside, but for mass billing its not very good. Sure, if you are billing customers with IT degrees, they can easily navigate in the ModernBill system and deal with its silly invoice system. But if you are dealing with a normal billing department, expect tons of people who have no clue how to pay you, or who are only accepting PDFs, meaning you need to do it yourself.
If you have a $500/yr budget, I'm assuming you have a big enough company to have trouble with this. That budget is too small, by the way. MBill leased is 24/month, plus the hours upon hours of coding and struggling to get it working. The labour cost puts it well over that. And Canadian dollars? Wow. Im dreaming if I thought MBill could do that. Although they *constantly* say the feature is coming soon. Same with PDFs. Its been "coming soon" since I think 2003. (Just check their message boards.)
We have zero problem with modernbill, its all running and setup. Our invoicing system bills automatically, The clients do not even have access to it. Its just for tracking details really and quick cpanel intergration. But the budget seems fine, modernbill is $180? And if we baught two versions its under our budget. But we dont really want to look into client exec when their online domain doesnt even work.