@wmasssoprano
Prices vary depending on what you are seeing. Generally speaking, Broadway musicals are $49 per ticket, while Broadway plays are $43 per ticket. Off-Broadway usually ranges from roughly $19-$34 per ticket depending on the size of the venue.
While there are rare occasions where shows will post a last minute offer, it is most common for tickets to be put out a week in advance. However, the box office can list tickets on any weekday, so there is no rule as to how far in advance things are posted. It is common to see the the highest amount of offers go up on Monday or Tuesday, as box offices are looking at the sales for the week and decide then what days need TDF.
I simply don't have the funds to pay for full price broadway tickets nowadays (well, if I want to see a bunch of shows in the season anyway) so TDF is an essential purchase for me. One thing you should consider is that weekdays are listed more often than weekend performances, at least for Broadway. So if it is impossible for you to attend on weeknights, you may not find the same worth as others do.