Logistical software. They follow something called the "profit gradient", which basically means that they price to maximize the number of pieces sold times the net profit per piece.
This is an easy thing to draw on a piece of paper, but a little bit complicated to describe in words. The short answer is, we know that as Wal Mart increases the price, they make a greater profit per piece - if you graph it, it's a line going up. We also know that as they increase the price, the demand from customers like us drops - if you graph it, it's a line going down. Now generally speaking, in math, if you multiply the line going up with a line going down, you end up with a curve that looks like one bump on the back of a camel.

Wal Mart is trying to get to the top of the bump, which is the price at which the product of demand and profit per piece is highest. In order to find the top of that bump, they need data, and in order to get data, they have to move the price around and experiment through time. They need to know that at price x, demand is a, and at price y, demand is b, given roughly the same location.
Now we can all think of simple examples of this, but humans aren't simple, and as you can imagine, this gets complicated fast. For example, the "demand line" (we usually call it a "demand curve") isn't smooth, because people are emotional buyers. They'll pay $99.99 much more easily than they'll pay $100. They'll pay $300 for a winter jacket in November, but only $200 for the same jacket in April. They'll pay $1 for one, but only $2.50 if it's packaged in threes. They'll pay $30 for a tee shirt in the women's department, but only $20 for it if it's in the men's department. They'll pay $10 for sunscreen on a sunny day, but only $5 if it's raining.
Walmart has an EXTRAORDINARY model that includes all of these factors, builds curves, multiplies out and finds the maximum (the hump of the camel). The math is absolutely out of control, and they have mathematicians and computer scientists on staff. It's these guys who figured out that on a national level, the golden price for a top 40 CD is $9.72. Have you seen the $9.72 CDs at Walmart? They are so sure that this is the key profit price that they are hammering the record companies to comply.