I will add all the stats from my runs here and update it regularly:
After first 10 runs: very lucky so far with meteor fragments, got 3, chance for Dragon is 10% already
After 20 runs: first nuggets and bones arrived, one Seadragon again, still holding on 10%.
After 30 runs: chance of encountering a Seadragon rises to 13,3%. Common Herbs still at 100%.
After 40 runs: got a couple of Dragons so the chance now rises to 15%.
After 50 runs: 43rd run brought a first time ever 10 drop of herbs, Dragon chance now at 12%.
After 60 runs: Seadragon at 13,3%.
After 70 runs: Seadragon up to 14,3%, common herbs still at 100%.
After 80 runs: I am still getting lucky, Dragons now at 16,3%.
After 90 runs: Dragons down slightly to 15,6%. Got some more dragon bones.
After 100 runs: Dragon chance down to 14%.
After 110 runs: I hit a wall, no Dragons since run 84. Chance now down to 12,7%.
After 120 runs: Dragons back up to 13,3%, lots of scabards in these last 10 runs.