Hello Brittany. How many cubic feet of negatives do you have? With my last employer, we purchased a used frost-free freezer from a biomedical company for $800, but that was for a sizeable collection of sheet film. A small budget was alloted each year to account for maintenance, but all-in-all it was a fairly inexpensive endeavor. Scanning the negatives was costly, but we were able to secure grant-funding to complete that part. I wrote an article about the process, if you’re interested in reading it: http://docs.nrm.org/portfolios/2000s/2007_Summer_complete.pdf (pp. 14-15). The method we used to freeze the negatives was developed by a conservator, and is available for free download: http://wilhelm-research.com/subzero/cmi.html.
It has always been recommended to me to retain the original negatives as they are the master copy of that image.