While it may be difficult to estimate how much money you'll spend in your travel destination, you can do some research and make an educated guess.
For instance, although it's possible to do Iceland on a budget, it tends to be pretty pricey if you're not careful — a burger can easily cost $25, for example.
So, make a travel budget: Try to get a set amount of cash out for the week and limit yourself to that. I like to think in terms of my income. If I make X amount an hour or day, that'll pay for X, Y, and Z.
Similarly, it pays to get creative. If you forgot to bring a sweater and there's suddenly a temperature drop, thrift stores will help save the day. Plus, they're a great way to get local fashion for less.
Since I tend to live places longer, I often end up trading American clothes with a local I befriended (they appreciate it as much as I do), which is another cost-effective option.