We don't budget but we do watch costs. Since gas and CGs are the big expense we plan ahead before we pick a CG. ie if one CG is 5 miles off of our route but doesn't save us more than 4 dollars (todays gas price about) we won't go their.
For weekly or monthly sites, one thing we look at is how far to town to get groceries but there are other things to consider.
At 10 mph, and 3.50 or so for gas, we try our best to save some money. But at the same time, we knew going into this that those two items would be our biggest expense.