Started a test to see how town relation effects buy and sell price of Armour with a town, same item used for all buy and sells, trade skill is at 10, was taking forever to get relation up by just buying drinks so I started doing quests which is why not all the relation ship numbers are included, if the number is not included I did not test it.
Buy price 29186
Sell price 10102
Relation 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
Buy price 28961
Sell price 10327
You earn roughly 2.2% more money per sale over the base price, in fact for all the following figures you make either +225 or +224 selling the item over the previous level.
Relation 12, 15, 16, 17, 19,
Buy price 28737
Sell price 10551
Relation 27, 29, 30, 32.
Buy price 28512
Sell price 10776
Relation 37, 38, 39, 40, 41, 42, 43, 44,
Buy price 28288
Sell price 11000
Relation 45, 46, 47, 48, 49, 50, 51. 52, 53, 54,
Buy price 28063
Sell price 11225
Relation 57, 58, 59, 61, 62, 63, 64, 65, 66,
Buy price 27839
Sell price 11450
Relation 67, 68, 69, 71, 72, 73, 74, 75, 76, 77,
Buy price 27614
Sell price 11674
Relation 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88,
Buy price 27390
Sell price 11899
Relation 89, 90, 91, 92, 93, 94, 95, 96, 97, 98 ,99
Buy price 27165
Sell price 12123
Breakpoints appear to be at multiplies of 11+1 so -> 12, 23, 34, 45, 56, 67, 78, 89, you cannot reach 100 relation.,
Percentage of money saved or gained for buying and selling after getting relation up to 89+
6.9% cheaper to buy items over base price (0 relation).
You receive 17.39% more money for items sold over base price (0 relation).
Best way to think of it is, for the first point of relation you get and then every 11 points after that you can sell stuff for roughly 2.2% more than you could before and buy stuff for roughly 0.75% cheaper.
Amount of money saved for purchases is very small and not practical for cheap items, for lordly items it would amount to massive savings but being unable to predict where items you want to purchase will appear I'm not sure if it's practical.
For items sold the gains are alot more impressive and if you say captured a central city like Dhirm and kept it for yourself and used it to sell all your loot you could stand to make significant profits for your efforts over time. But do bear in mind that it's only 17% more money at 89 relation that means an item worth 100 denar would sell for 117 instead so you would have to be taking the long view and bulk sales into account.
Thier is no point in taking relationship with a town over 89 as that is the final breakpoint and no more gains can be made.
I would like to add that you will have a relation ship of 1 with any town you buy a round of drinks in or won a tournament in which almost everyone does for another increase of the same amount you would have to get another 11 points of relation.
Having trading maxed out and no relation will get you 34.6% of your buy price back, maxing town relation and trading will give you exactly 44.6% of your buy price back exactly 10% more.
To add a little more perspective getting you trading to level 3 will give you almost the same sales/buy bonus as working your relationship with a town up to level 100.
Odd note, this was the first test I started and I attempted to do it using a legit character by not saving and reloading when I ran out of money, this was taking way to long and kept getting interrupted by an on going war, additionally I already had relationship of 7 with the town so data was missing. So I created a new cheated character. But the data I obtained with the non cheated character was trade at level three and the data it provided me does not match the results of the trade 10 character. Breakpoints for a trade level 3 character (Started at relation 7) were 9 and 14 as opposed to the above 12 then 23. So this could be more beneficial to a non trader character or maybe my first test was messed up for other reasons. If someone wants to start a character with no trade skill and obtain the data be my guest and I'll add the info to this post.
Worth all the effort or not? On one city maybe if you take it high enough, depending on your play style, you decide.