Train cost

The easiest site to get this type of information from, as far as I could find, is Cleartrip.

There is also the official site of India railways that has a pretty difficult interface, but if you have the patience to wade around, you'll find interesting stuff.

And the Seat61 India section is just all around awesome if you haven't already seen it.

Cleartrip is simple to use and I believe the only site where you can buy tickets with a foreign (meaning non-Indian) credit card. It takes some perseverance, but if you follow the instructions on India Mike here it is possible. The trains get booked up very quickly, so it's a good idea to book as soon as you know where/when you want to travel.

YES - the trains book very quickly - trying to book even 2 weeks in advance is often impossible.

Don't know if the links say so, but there's a nice discount for over-60s.
Plus, you automatically get a lower bunk wherever possible, unless you specifically request an upper one.

Yes, I forgot that - it's 60 for men and 58 for women.
Cleartrip does ask if you're old enough to qualify and adjusts the prices for you.