Thank you all for your overwhelming interest on Golden Fleece!
When I designed the card I thought of course a lot about its cost. What I did was to look at situations in which I can buy it very early; e.g. 5/2 opening or hitting $5 in turn 3 or 4. In the presence of any of the powerful $5 cost cards I would likely prefer those over Golden Fleece. If I would be in the situation to buy Gold or Golden Fleece, I would definitively buy the latter more often than not. If there are strong $6 cost cards on the board, I would prefer them. With those comparisons in my mind, I decided to give it a $5 cost. I am sure with this cost it will be a competitive and attractive card, maybe not bought very early, but for sure in a lot of cases later on. I hope that I will have the opportunity to play Dominion in real life soon again. If so, Golden Fleece will be one of my first Fan cards that I will include in games. This will likely tell me more about its strength and about which cost is better suited. It is easier to start with $5, instead of $6, since I expect to see the dynamics of the card much more often. If it turns out that Golden Fleece causes a flood of Coffers too often, well then I guess it will have to cost $6.