The Higher Cost Will Pay Itself Off
Solid-state disks are considerably more expensive than hard disks, which is likely the biggest barrier for most people. 128GB SSDs will generally run you about $US200 and 256GB will run you closer to $US500. While this cost might feel prohibitively high, it's not as bad as you think.
Let's take the higher cost of $US500 for example and say you've purchased a 256GB SSD to upgrade your laptop. Straight out of the gate you have a leftover internal hard drive that you can either sell for about $US50-75 or turn into an external. Being optimistic, you're down to $US425. That's not a big savings overall, but if you upgrade your laptop every 1-2 years you can keep using the same SSD. When you buy a new laptop, remove its hard drive and swap it with the SSD in the old one. The old laptop you're going to sell will now have a larger capacity hard drive and you can charge a little extra when you sell it. Over three sales you'll probably make an additional $US100, bringing us down to $US325. When the time comes to upgrade to a new SSD, you can sell the original SSD alone or with your old computer. Mark up the cost of the machine by $US325 and you've covered your original costs. $US325 for an SSD laptop upgrade is a pretty good deal. Of course you'll need to buy a new SSD, but you'll have made back your money at this point while using your SSD for several years.
Note: SSDs don't last forever and performance can degrade over time with some models, so make sure you get a high-quality SSD that's rated for a long life if you're going to do this. You can use a tool like SSD Life to keep an eye on things.
Size Doesn't Matter (Sort of)
After asking how much hard drive space you actually need in your computer, I came to realise that you, our readers, are a bunch of insane hoarders. That's OK! I like to hoard stuff too, but I'd never pay extra for more than 256GB of disk space. When you have more space, you're most likely going to use it. This puts you in the habit of storing useless files on your computer that you either don't need or rarely need. It's like living alone in a two-bedroom apartment just so you can use the extra bedroom as a storage closet. For the most part, you can get rid of a lot of the files you think you need.
For some of us, we have a massive collection of crap we find comforting. For others, the space is necessary for large media projects. I fall into both of these categories, but external drives solve the problems far better than trying to manage huge amounts of data on the main internal drive. First of all, you get the benefit of virtually limitless space for your stuff. Second, you can organise your stuff by the drive if that helps. Third, if you need additional protection offered by, say, RAID, you can get that with external drives whereas you're not going to get that with a laptop and some desktops. Ultimately, the size of the internal disk in your computer can actually be bad for you after a certain point. For me it's around 250GB (I only use 200GB but I like to keep about 50GB of space free). For some of you it may be as little as 128GB, a size where SSDs are significantly more affordable.
In the end, the size limitations are going to be better for you. You can horde your crap on external drives, stay more organised, and reap the benefits of a super-fast solid-state disk in your laptop or desktop computer.
When you decide to take the plunge, be sure to read how to take full advantage of your solid-state drive.