I managed to get to Trade level 300 from 265 in 6 hours of playtime after the 1.5.10 beta update. Unfortunately, as prosperity was running amuck for the playthrough it costs over 4 million to buy a town and near 6 to buy a high-prosperity town. Even Castles with 1300 prosperity were running over a million. But since the devs made the change to prosperity it may be worth another trading playthrough to make it a viable way to get fiefs.
I also changed my trading strategy which I think helped a great deal. Instead of solely relying on horses and looking for the best trade deals I started taking advantage of the colours in the trading menu. I found it took too much time to reference pages of min/max buying prices and after over 1000 days in-game the economy had shifted away too much from what other people had experienced in their playthroughs, as well as my own experience at the beginning of the game. So I ended up trusting the colours to say that an item was selling for less than the global average and I bought it regardless of what I personally thought it should be worth. I also sold anything that was in the dark-green (on my own side) because it meant I was making a profit on the exchange. At times I was buying 30k worth of goods at a time but I was also very often selling over 40-50k at a time. I also bought all of the mules and sumpter horses under 50 denars so I could carry the upwards of 80k weight but I was selling off horses if the price was at least 100 denars.
I found the key to this strategy was moving through the map one city at a time, but not revisiting a city before I had been to every other city, as essentially what I was doing was stabilizing the economy and bringing goods prices closer to average in that particular city; it needed time for the local economy to go back out of balance.
Another key was to not spend much time looking at prices when buying and selling and instead just trusting the numbers. The game knows what you paid for an item, so if you sell when its dark green you are making an adequate profit to level quickly, even if that isn't the best possible deal for that item. Speed is crucial. I would always sell off all dark green goods first, then buy dark green goods after. That way if I was selling something below the global average, even if it was making me profit, I was selling it back to myself (retracting the deal) so that I could make a better deal somewhere else. At the end of a trading session, the entire "city" side to the trade menu did not have a single dark green item for sale.
I should also note that it took roughly 2 times around the whole map to get from level 265 to 300 and I made about 900k denars in that time frame.