As you say, getting more hard drives is going to cost you a fair amount. This also doesn't solve the problem of losing the drives, theft or something like damage in a fire.
As Dataman said, online storage will still cost a fair bit for that amount.
However, I did read about a service called Backblaze, they have optimized mass file storage to make it mega cheap for them to offer. They offer unlimited storage for a monthly fee. Of course, a lot of people offer unlimited things online when in fact they're not. But from what I've read this service is pretty genuine. Their business model works on the fact that most people will use a small amount of storage and some people will use a lot, overall it equals out as them staying profitable.
At 10 terabytes, you'll be getting your monies worth, in fact you'll probably be one of the ones they make a slight loss on. Take a look at the pricing, this is for unlimited storage.
$3.96 a month for 2 years ($95 total)
$4.17 a month for 1 year ($50 total)
$5 auto-bill each month http://www.backblaze.com/
They have less features than other services like dropbox, but this is a good thing, it means you get a better price because you're not paying for stuff you don't need.
Upload everything you have (be warned it's going to take ages, Im hoping you're not on dial-up
). They will encrypt it, compress it and store it. Then just forget about it, unless something happens to your drive.
I wanted to test out the service, but I don't even have enough data to upload lol.
If you do decide to ty it out, let us know how you get on