The old days are over. If you want to take a written test it will cost you big money (few thousand) and in order to take it, you need to go through the approved course. An airline ground school does NOT count as the course.
If you hold an expired written, it is no longer valid is my understanding. Anybody who took the test by July 31 2014 has 2 years from that date to take an ATP checkride, otherwise you're SOL and need the course.
This is why I'm very curious as to what the pilot pool will look like Aug 1st of next year. Anyone who took the test last July only has a year and a few months now to take the checkride, but they'll be the last to get an ATP under the old system. From here on out anybody who gets an ATP will have to have gone through an approved course, so unless you're hired for an airline that provides the course prior to ground school, you won't be getting an ATP for cheap like the old days.
Edit: Here's the kicker, a lot of places still don't even have FAA approval after nearly a year, so I would really like to see the figures of how many people have taken the ATP written in the past 10 months.
Here's a few to give you an idea of cost (I'm finding $5k to be the average)
https://atpflightschool.com/atp/ctp/index.html
http://www.sportys.com/pilotshop/atp-certification-training-program-atp-ctp.html