I just flew Delta in September 2015 (JFK to Madrid and Barcelona to JFK). On the first leg, the upgrade to Economy Comfort would have been $120; on the second leg, it would have been $80. Both of these prices were at the time of booking. For at least one leg (I don't remember all the details), I was also offered the upgrade at the time of check-in, and I remember the price was slightly different; not sure if it was higher or lower.
Of course, I was also offered an upgrade to business class - at substantially more money.
The only way to know what upgrades are being offered and at what price is to do a dummy booking on the airline's own website; you will be offered the upgrade options before you enter payment information, so you don't have to commit before seeing the options.