Ok I decided to try the 16 card bosses to see how much effort it was to do them so when rewards are bettered I'll know how worth it it is.
I did this on Assassin, which for anyone reading who wants to try it I HIGHLY recommend it because it saves ALOT of time or wine.
Card Gathering
Costs 1,344,000 to make each card, takes about 5-10 mins to do.
2 79 Card Bosses are easy enough, takes about 20 mins to kill the two of them (Most of it is running through and clearing mobs).
4 89 Bosses are easy enough, 20 mins per instance to do tops, they are at the beggining so its easy to access.
2 99 Bosses, these were a challenge, took an hour between the two of them, but I also was doing bh in Abba.
2 God Valley bosses, these are a pain, yes they are easy bosses but it takes about 30 mins to get up to them, plus you need a good GV squad and unless you successfuly finish full noone really wants to do the card bosses (you need to finish wave 3, then lose on wave 4 (or others). So an hour or so on average to do, but 2 hours if you do a full GV which is more likely.
3 TT 3-3 Bosses, Took me 1 hour, any other class would be 2, it is REALLY annoying and Crystal Terror's stun and amp you need to be a very high defence tank (seeker) or have a cleric with you. Costs 157,500 in subs.
3 Lunar Bosses, Takes 2 runs, you need to go two seperate pathes to get the 3 bosses, first one is in first valley, which is easy, but the second one is in the valley of killing only the left tombstone and the third is in the valley of only killing the right tomb stone, costs 6m just for entry twice, can cut it down if you do it as a group with people and split the costs but once again finding a squad isnt exactly easy. 1 hour if you dont have a seeker to clear mobs (which luckily I do).
So in total the costs to get to hammerfell
Probably 5-6 Hours of your time, so a day or two of playing.
7,501,500 in coins, the coin drops of the few mobs killed is most likely used up in repairs.
Total cost if your not a sin and use wine (otherwise add say 2 hours on the time to do it) is 8,026,500.
So it is infact ALOT of work to get to hammerfell, personally I think (I know its not up to me, just suggesting) that the big reward should only be from hammerfell (but do add in mats for ec trade from the other bosses), since the effort is to get to him, its easy to spam the weaker easier to get to bosses, so a reward worth atleast 8m and 6 hours of your time, if not more to add incentive to do it, would be great

.
Call it 2 GVs in 6 hours, thats about 150 EC and 150m xp (and alot of cash), so just suggesting you work off giving a reward similar to that

. (I just guessed the xp amount, but 75 EC is what i average in GV)