I take a 100 flat rate for replacing iPhone 4/4S screens, 150 for iPads, 20 for back covers. I used to take 40 for replacing iPhone 3G/3GS digitizers, but now I take 60. I also adjust the price if I see that the person has a lot of money. I give rebates to friends at times if they ask.
Other repairs depend on the situation. For macbook repairs, i don't give a fixed price, because usually I can get more out of it like this. Then, I charge 100 hourly. If they ask if I really repaired something, I just showed them the video that I took (pro tip: take videos of all the repairs you do and post them unlisted on youtube, and send a link to the client if he has questions).
You should also get parts directly form a Chinese supplier. When I buy parts from them, they ship it right away, and they arrive in 2 or 3 days after. It's much better to buy in bulk. Usually I buy in the 300-500 range - if the volume is lower, the cost of shipping is unjustified, and if it's bigger, there's the risk that demand for parts will go down sometime is more significant.