Travel budgets tend to grow through lots of small additions, so checking every expected expense beforehand is worthwhile. Documentation can be one of those categories that gets forgotten when planning the main costs. If a land crossing is involved, researching Mexico Land Card Fees can help when putting together the budget. I’d leave some flexibility as well, since transport, accommodation, or other parts of a trip can sometimes change unexpectedly. Having a realistic estimate before departure makes spending decisions easier once the journey begins. It also means there is less chance of having to cut back on planned activities because an overlooked travel expense appeared at the last minute.