Get a quote from a few local storage companies - it varies a lot depending on the company, where you are in the country, size of unit and how long you want it for.
Consider the value of what you're storing vs the cost. My dad was renting a storage space for loads of junk and Christmas decorations etc, he was spending £150 a month and had been using it over a year when we found out. The stuff in there probably would've cost less than £300 to replace if he'd thrown it all out and bought again later! Obviously furniture is more valuable but if squeezing in a few inexpensive bits means the difference between a small or large unit it might be better to throw out/give away the lower cost items.