I'm currently offering 4 different annotation plans. The most inexpensive one starts at $20 per game. I've edited a 5 pages .pdf document which fully explains my annotation services. This document contains a comparison table with all the information that you'll need, including fees. Since I get requests from different countries all over the world, the pricing table will be slightly different depending on your what country you live in.

Engines are mainly useful to calculate the main lines of a tactical idea, but THEY DO NOT:

• provide comments on chess strategy: no plans, just moves
• give clear explanations about why a move is good or bad,
• point out your strengths and main weaknesses in your play
• suggest cool opening ideas which might actually work against the opponents that you play
• answer many of the questions that you have about your play, your opponent, your training technique or a particular position
• provide psychological tips that can improve your play
• know how you feel when you play against another human being...

Not often, but it can happen, sure, I'm only human. A good student will review my comments and find those mistakes!

It could take anything between one and seven days. Sometimes I spend large ammounts of time annotating a game... quality work takes time!

I definitely do. You'll have to fill in

A pgn viewer is basically a program that allows you to follow a chess game with a board displayed on the screen, and a panel on one side displaying the chess moves, variations and notes. The moves are displayed on the board, making them a lot easier to follow.
Some of the advanced pgn viewers such ChessBase Reader display graphical elements also (coloured arrows and squares). Do you need one? unless you want the notes in .pdf format, yes, you do. The good news is that many of them are free. If you're on a window computer, You can download Chessbase Reader (recommended!) from Chessbase's official site: ChessBase´s Download Page
If you have an Apple computer, I'm told that one of the best pgn readers is ChessX, which is also free. You can download ChessX from its official sourceforge website.
If you're using a mobile device, you can google "pgn viewer + whatever device you're using".

I explained the limitations of computers in a previous question. Computers have gone far, no doubt, but they can't annotate games like a human being: sure, they have a tremendous calculating power, but they can't suggest plans and explain them in a human way and they don't normally tell you why a plan is dubious, nor can they provide practical advice or mention any of the important psychological aspects of a chess game. Also, I normally adapt my comments to my students´ level so that they're easier to understand. Sometimes I also engage into calculating interesting side-lines, not out of fun, but because I think the chances of someone actually playing like that are actually quite high.

Yes, of course. Sometimes a student might want to get notes to one of his favourite games played by famous Grandmasters, or perhaps he's preparing a particular line in a well known opening and there's a critical game that he needs to understand really well and so on. .

It depends on the plan that you buy and the length of the game. On average, if you were to print out the pgn or chessbase file, I'd say that you'd get a minimum of at least 2 pages of analysis, often more than that. (except the "Chess Informant Style" annotations)