RAID is not backup.
Now that that's out of the way, if you have 15 servers which only hold content, and each one is 250 GB, it's time to ask yourself some questions.
0) Should the data be centralized?
Unless you just happen to like managing the storage on 15 machines, you should probably shoot for pooled, managed storage. This does come with a cost, though, Storage is cheap. Managed storage is expensive. If you don't want to (or can't) manage it centrally, then you need a tape solution. The cheapest solution would be one server with a large amount of disks (in a RAID configuration) attached to a pretty large tape changer (ideally, since you don't want to manually change tapes every day, I assume). You could also get 15 tape drives and attach each one to a server, but that's dumb.
1) What is your data retention policy?
In other words, are you going to keep the data forever, or for a limited period of time
2) What is your size delta?
How much does your data change per day? That needs to be factored into your future storage plans. Equipment purchases are not just IT related. Accounting needs to be factored in. If you depreciate your purchases over 3 years, you need to purchase storage that will last you 3 years. Do the math or pay the price later.
3) Where are you going to put it?
15*250=a lot of data, as you mentioned. You've got to figure out where you're going to put it. If you want it to be "live", you've got to get a storage array of some sort. If you want to back it up to tape, you're going to need a tape changer attached to a server with some big storage.
4) How much of the data is a copy of the other servers?
If you centralized the storage, you have the opportunity to invest in a storage array that has "data deduplication", which saves tons and tons (and tons) of space. Essentially, if a file over here has the same data as a file over there, the data is only stored once, and a token is stored instead in each place which is smaller than the original data. Solutions that provide this are expensive, though.
Please tell us more about the current network topology, data characteristics, server specifics, and whatever else you can.