Very useful information. Thanks beezneez. I see that $38.10 will buy me 16 rides. Just to clarify, am I correct in assuming that a "ride" is defined as any single, one-way trip? Or can you make more than one trip on the same ride within a limited time frame (e.g. 2 hours)?
Think of it like this: you have to swipe your MetroCard when you enter a station, so that's when the $2.50 is deducted from your card. Transfers are free if two lines stop in the same station. There are also some places where stations that are really near each other will have a Free Transfer, which you can find on the subway map by looking for black lines between stations (see 14th St Bryan Park BDFM station and the 5th Av station on the 7 line). Theoretically you can sit the entire day in the subway for $2.50.