I did a bunch of fare comparisons when they put the eUpgrade stuff online looking at the difference between lowest-fare T+ and M class. I came up with about $45/credit as I recall. But, as Jasper2009 points out, it will depend on how many you have, your routes, etc. I think someone with 16 credits looking at R9 and a YYZ-SYD flight would pay more than $45 for one more credit.