I'm the same guy in both posts.
Nowhere in Latin America will you be asked for 'road use fees'. You import yourself and your vehicle, and that's it. You might want to cross toll roads (Mexico) or take ferries or go into National Parks, those cost money.
Mandatory insurance happens, yep. Also in Central America, mandatory fumigation at some borders. It's a few bucks.
Keep the bike! That's fine. Store it down there, ship it back later, or do another ride. But going back to the US with a SA-bought bike is asking for trouble. Re-registering it will run afoul of the DOT, which requires things like instruments with miles per hour on them or other things like emission norms in California. Not to mention you have to 'import' the bike, which will not be free.
DL650s are great, if a bit heavy. Just do all the good work on it and check online for the most common issues of things that break. Bring spares for those or fix them if they are close / in the engine.
On the regular? Depends on the country. Costa Rica and Panama are expensive. In Guatemala and most of Mexico we only spent a few bucks a day excluding gas. I remember rolling into Nicaragua and thinking 'fuck, I have to stop splurging on these $3 hospedajes'.
I have no idea what I spent on fuel. I'm messy with finances. Big other expenses are trips, You'll go to Ecuador and meet a hot backpacker girl at a hospedaje or hostel and she's going to the Galapagos. Hey, come along anon, I'll suck your dick while you look at weird beaked finches. Oh shit, that's over a thousand dollars.
No, seriously, you'll want to spend a little to go into national parks, join experiences with other travelers, perhaps even buy something cool (I got some weird ass guatemalan pants). That'll be your expenses behind the big two: lodging and gas. Food is cheap almost anywhere, and where it's not, you can go to the almacen and just get groceries to cook yourself. Or get street food. Do pack diarrhea tablets.