If you heirloom your weapon 3 times, u will get +3 to main damage, +2 secondary damage, most weapons get +1 speed and some get more weight, and u dont have to pay any extra upkeep for that, even tho it shows different price for it ingame. On armors u will get +5 armor when heirloomed 3 times, wich means if u loom ur head, body, hand and leg armors u will get +5 head and +10 body and leg armor, and it wont reduce ur movement speed or increase ur upkeep. Horses get + to all their stats and shields get buff to all other things exept size.
All in all if u have all ur equipment fully heirloomed, u can do better. And the prices are what ppl pay for, if u go whole generation(1-31 and retire) with setup cost under 20k(~15k), u can make allmost 500k money in that generation.