You should get more gold on faster speeds, not less (although it's probably set up the other ie. wrong way round). The other thing to consider is that the return scales with the size of the foreign settlement. If you give "too much" for prehistoric settlements sized 1-3, the amounts as cities reach sizes 50-100 really get out of hand, especially if they scale "linearly" or above, and as better traders get better base rates. So it's tricky, but I have to say that imo he should get a minimum of 1 gold for every turn the trip took. And if that means a Renaissance trader to a size 100 city gets too many billions, then the effect of population (and maybe even distance) needs scaling back.