Just got back from three weeks in Colombia. Here are notes based off my travel in Jan 2016. I'll be comparing prices to US dollars.
The exchange rate is approximately 1 USD = 3300 COP at the time of this writing. To estimate prices, you're better off just dividing any price you see by 3000 to get a fair estimate of its USD price after any fees you may incur.
You'll easily find a secure cash machine in Colombia. I had no trouble locating one in these destinations: Cartagena, Barranquilla, Santa Marta, Bogota, Pereira, Salento, Medellin.
Some machines had a max withdrawal of 300.000 COP, and the highest withdrawal limit I saw was 600.000 COP. You can perform consecutive withdrawals if necessary, but your bank's fees will stack. Bank of America charged me a 5 USD fee per withdrawal for most machines.
I recommend going with the maximum withdrawal amount, keeping a third of it on hand and tucking the remaining two thirds in a safe place. It is a withdrawal of 200 USD at most. This will get a single person by for several days to a week and will keep withdrawal fees few and far between. It will also save you time in repeatedly finding an ATM. You can then leave your ATM card safely locked up in your hostel.
Cash (Efectivos) vs Credit (tarjeta de credito)
You'll need cash on hand at all times. That said, don't bring too much of your home currency: 50 USD is fine to get you through the initial days. The rate at currency exchange vendors seemed to be about 1 USD = 2800 COP. That's a 15% charge! Awful.
Use an ATM soon after you land. If the max withdrawal is 300.000 COP and the fee is 5 USD, then that's about a 5.5% fee. Again, I recommend you go for the max withdrawal, but keep in mind that you do not need more than 100.000 COP on you for a full day's needs and then some. If you plan to shop, maybe twice this amount. If something is pricier, you probably can pay with credit card.
Credit card was the most effective option for me. Use this option whenever you can. My card had a 3% fee. Stick with Visa or Mastercard, as I doubt the support for Amex or Discover down there. The clerk may ask you for how many counts or cuotas you want... This is like a form of installments. Just say one count. The receipts include lines for you to write down info like your phone number but you can ignore it. Almost everyone was cool with only my signature.
A local meal will run you between 4000 to 10.000 COP. The fanciest meal will be 40.000 COP.
A taxi within a city will cost you between 5000 and 25.000 COP.
For more detailed info, check out this resource. It cites costs in Bogota, which will be one of the more expensive locations.
Keep a color copy of your passport on you. Bring a student ID if you have one too. That's all you need on your person. The student ID really helps at some of the major tourist destinations.