Yes, the stumps can get sore from quite a few things, the most common are:
Pistoning, when the residual limb comes out of socket and back in, because it’s not firmly held in place.
Belling, when the limb is snug on and around the knee, but rocks back and forth inside of the socket. Think of a clapper inside of a bell.
Sweat, pure and simple… it sucks. My stumps are encased in 6mm of silicone, then a carbon fiber shell. Theres no room for breathing, so I sweat my ass off. Which can lead to infected hairs, heat rash and the contact points on your limb being rubbed raw.

The height addition depends on the runner. Since there is so much spring added with this type of prosthetic (We call them Cheetah Legs) you typically add an average of about 4 inches to compensate for the bend.
Stride is also dependent on the person running. Like anyone else, you could be a short choppy runner, or long, big strides.

The number one thing all lower limb amputees work on is core training, to strength our core muscles to compensate for the shift in balance. It’s a bit awkward at first, but like anything we do in life, with a little practice no one even notices I have prosthetics anymore. My gait and stride is near perfect.

Yes, all the leg components are expensive. Most of the metal is titanium or stainless steel, and it’s mostly encased in carbon fiber.

As far as cost goes, I pay absolutely nothing. Everything is paid for by the Govt. since I lost my legs while serving over in Iraq on active duty.

It takes a lot of time and practice to get used to a set of legs like this. You have to take in consideration of knee strength, bone density and muscle movement. I can’t say I run faster / better / farther than I used to, but they allow me to work at the best of my current abilities.
These legs provide upwards of 85% energy return through the flex of the leg itself. The human ankle provides 200%.

And no problem about the questions, not to many people get any straight answers about stuff like this. I know people are curious, I get the stares and pity smiles all the time. But I would rather answers questions than have people live on assumptions.

So fire away. =)