Three. And an extra hard drive. That's how many it took to safeguard my master's thesis.
Exactly...you're complaining about the expense of the disk, then complaining about the expense of NOT having a backup. Skip a meal, spend the extra money.
I had a client not two months ago learn that lesson. Didn't back up enterprise-level operational data because it was "too expensive" (at most, $300). Lost their data, took $100k and six weeks to restore from scratch. BAD risk analysis, whether it's your school project or several hundred million dollars worth of financial data...