I´d sure allow more than $20 per day. If I´m moving steadily, I spend that much on gas alone. Borders are anywhere from free to (so far)$33, and again if you´re moving fast you´ll cross one border every couple of days. Hotels range from cheap ($6) on upwards, but I find myself averaging $15 for cheap places with secure parking. Food: also variable, from $5 to $25 per day.
You add it up. But don´t forget to add other bike expenses: I´ve got a rear tire which mysteriously wore out at least twice as fast as I ever considered possible (Kenda 270 almost bald in 4000 miles!). I don´t know what a replacement will cost, but I´m about to try and find out, and it´s not going to fit into any $20 per day budget.
Hope that helps....but don´t forget, all of the above prices depend on, basically, everything under the sun.
(from Danli, Honduras)