Just like the stock market, the currency market also changes every couple of minutes, so your rates will never be static throughout the day or week. Currency exchanges are not too tricky to understand, but it certainly requires an explanation to grasp fully.
Assuming you're from the United States, when you're going to Mexico, you'll want to understand the exchange rates from USD (dollars) to MXN (pesos). This will show you what $1 USD is valued in today's market as Pesos.
When you're coming back from Mexico to the United States, you'll want to understand the exchange rates from MXN to USD. This will tell you how much your converted Pesos are worth in today's market as Dollars.
When exchanging cash, you agree to receive that day's exchange rate.
Values and Rates
On average, you will get the currency exchange as such:
for every $1 USD (dollar), you will get about $20 MXN (peso).
for every €1 EUR (euro), you will get about $24 MXN (peso).
for every £1 GBP (pound), you will get about $27 MXN (peso).
Every day, the values change. You can use Google to see how much it costs to buy and sell MXN. Buying MXN outside of Mexico will be cheaper than buying MXN in Mexico.
If you are using a travel credit card, you will get the daily exchange rates with your bank, which is generally the same of the daily rates you can find on Google. Travel credit cards don't have a foreign currency exchange fee and are the safest bet to getting the proper exchange rate at all times while on your trip abroad. Click here to learn more about travel credit cards.
Exchanging at your local bank or exchange offices
Understand that you will never get a 100% exchange rate unless you are using a travel credit card.
When you exchange cash, you're not exchanging currency, but rather buying currency. You will almost always pay certain fees, whether they are disclosed or not. Your goal when converting cash is to get close to 100% value as possible to what you see on Google's currency value.
When you're not in Mexico, buying Mexican Pesos will almost always be cheaper than buying it in Mexico. What I've found to be best is to contact your local bank or local currency exchange office prior to your trip and ask to purchase Mexican Pesos.
If we assume $1 USD can purchase $20 MXN:
Spending $100 USD (including fees) into buying $2000 MXN is 100% value, which you will not get with cash exchanges.
Spending $100 USD (including fees) into buying $1900 MXN is 95% value, which is excellent value. I'd accept this offer instantly.
Spending $100 USD (including fees) into buying $1800 MXN is 90% value, which is fair value. If I knew this was going to be my best offer, I'd accept it.
Spending $100 USD (including fees) into buying $1700 MXN is 85% value. I would look elsewhere because 15% difference in currency is too much.
Anything below 90% value is a scam or bad value.
Exchanging currency in Cancun
So, here's the thing... You shouldn't be exchanging any cash in Cancun with an exchange office or even your hotel. You will almost always get a terrible exchange rate or get scammed.
Instead of taking cash with you to exchange in Cancun, keep your cash in your debit card and withdraw it as Pesos at a bank's, and only a bank's, ATM. You will get close to 95% of the value of the daily rate, which is an excellent value.
Note that some ATM machines will show you a screen asking if you want to use that ATM's exchange rate or fee. You MUST decline this. You will still be able to withdraw your amount in Pesos. Another thing to remember is that most debit cards will charge their own personal fee, such as foreign ATM fee, which might give you an overall total lower value of your exchange when factoring in.
The most reliable ATM machines to use for withdrawing Pesos are the ATM machines belonging to big banks, such as:
Santander (You can find a Santander's ATM machine in Plaza Kukulkan Shopping Center)