Basically you have 2 choices since uploading the files is not an option.

1 - Some type of tape backup system, I work with tape silos every day, and while they cost a bit for the initial setup, buying new tapes is relatively cheap for the amount of data storage you get, making it well worth it in the end.

2 - Backup to hard disks which are then taken off-site daily.

If you want to have a series of backups really your only choice is to use tapes, having multiple sets of HDD's will add up quickly. However, if you are willing to overwrite your data each day then a couple HDD's is probably your best choice.

Iron Mountain, in the US, is a very good data storage company. I've never had any issues with tapes getting lost or anything, and I send off/receive close to 1000 tapes per week. Sounds like you are dealing with smaller scale then this, so just taking the backups home with you each night should be fine. Maybe invest in a small fire-proof safe for your house to store them in.

Also, yell at your bosses about giving you a tiny budget, 1500 Euros is nothing for how much time/money will be invested into restoring the data if it's lost/corrupted.