The short version: exchange rate volatility between US or Canadian dollars and Mexican pesos. Your closing costs and acquisition tax are normally paid in pesos, and the MXN/USD (or CAD) exchange rate is famous for its occasional sharp swings. Any movement in the rate between signing your Contract to Purchase and the closing date gets baked into your final closing costs.

Suppose your closing costs were estimated at $510,000 MXN when you signed the Contract to Purchase. At an exchange rate of 17 to 1, that's $30,000 USD.

Now imagine that by closing — or even a couple of days before — the rate slid to 16.5 to 1. The dollar figure would rise to $30,909, meaning $909 USD more than you'd planned. The swing can just as easily favor you: at 17.5, your costs would come in $857 USD under the original estimate.

That's why we usually ask buyers to send an extra $1,000 USD or more, scaled to the purchase price, as a buffer against rate movement. Whatever isn't needed gets wired straight back to your bank account.