Samsung makes some of the best consumer-grade solid state drives around and this $68 Samsung EVO 500GB model is currently marked down to an all-time low on Amazon. Remember to clip the 5% coupon code to save the extra $3.
It bears repeating that SSDs are a huge upgrade over a spinning hard drive for your computer, in terms of speed and reliability. This is also the easiest way to get more performance from your older computer.