Actually, you may be able to rent a pump for far less then buying. There are also some hospitals that offer short term (hourly) rentals, so you can stock up from time to time, but not actually take the machine home with you.
Warning: Some women just can't pump. I made enough milk to feed a regiment (the lactation nurses at the hospital where my son was born reeeeeeaally wanted me to be a donor, and I honestly would have loved to)...but the pumps just didn't work!!! I could never get more then an ounce...but they weighed my son before and after feeding, and he never took less then 20 ounces!!! (Should have seen that little belly expand). We tried off and on for months, but irritatingly, my boobs would not be fooled. No matter how much i wanted them to be. We went through everything from their spiffy ones to the 10 dollar cheap handheld ones to expressing. Zip. Nada. Zilch. Darnnit!
But I nursed not only my son, but my friends daughter when she went in for surgery. No problemo. Well, I'm not letting my ego get dented over a machine.
For emergencies (used twice...once for a date, the other for a midterm) we kept a can of enfamil lipil in the pantry. But it did also mean that my husband never ONCE had to get up in the middle of the night. Sigh.