Ok lets use a time based equation = how much time to get EC/FM

The Avg lvl 50 player could do 4 runs of Tour the Universe in 1 hour or $1.4 million

If you do the route from the wiki - there is also another rount where you only do a few blocks but get 200k EC and can do it 10x min or $2 million EC

Lets use the low end of $1.5 million

A good player with 5 toons could run Colony invasion 5 times in 1 hour min for 30 marks each run or 150 marks

so 150 marks = $1.5 million EC on a straight time trade - I could do the tour and get $1.5 million or I could do Colony invasion and hopefully get the same $1.5 million

So each mark would be worth 100k under these conditions

Given that you get some almost worhtless fleet credits back - lets discount that a bit to 1 mark = 75k

SO 1 Mark equals 75K Ec - less than that well you ar better off running Tour

Now trading Marks would be a good idea - maybe they could have a Fleet mark exchange??

Of course people who have lots and just want to unload would sell for much less - this is just a "Your Time" cost analysis