Buy a new 2TB drive, copy your data from your smallest drives to that one, then sell them at eBay.
until you have all your data on new 2TB drives
At the end you should have money for an extra drive.
If you like (and use Linux) you can start a RAID5 with one drive (+1 missing), copy your data onto it, then add another drive, do a reshape, migrate your data...
And at the end add the last drive to regain redundancy..