Internal hard drives cost roughly $0.33 USD per GB while an external may be around $0.50 USD per GB (some more, some less depending on capacity and where you buy). Depending how much you shoot for a client, with redundancy you are looking at just a few dollars for each client.
Lets do an example of a portrait shoot with a few different poses:
I'm going to need to generalize for some numbers in this example, but will bias on the larger side of things:
Say you are shooting a 20D camera which gives you on average a 8mb raw file. Each file requiring edits goes to a 48-bit layered tiff file averaging say, 150mb (three full layers) each.
Now lets say you keep 40 frames for the client to choose from and they choose say, 15 to order prints from. Each needs to be converted to a layered tiff. That is 320MB in raw files and 2.2GB of tiffs. 2.57GB in hard drive space for one client. Just for kicks we'll say it's 3GB for misc. additional items (such as jpegs to send to a lab).
With single drive, it is $0.99 for that one client.
With RAID 1 (mirroring) system it is $1.98
With RAID 1 + a Backup it is $3.48
You'll have to calculate specifically for your situation of course but If you can't afford that type of minimal cost in storage, then I'd say you have much bigger problems in your business than hard drive space. Something else to ask yourself is how much did you spend storing negatives and other related items when shooting film?
