Here's the stats on both:
Heavy Bastard Sword
weapon length: 105
weight: 2
difficulty: 10
speed rating: 98
weapon length: 105
thrust damage: 23 pierce
swing damage: 36 cut
slots: 2
Secondary Mode
5480 gold
Longsword
weapon length: 106
weight: 1.8
difficulty: 10
speed rating: 98
weapon length: 106
thrust damage: 23 pierce
swing damage: 36 cut
slots: 2
Secondary Mode
6660 gold
In short, for 1100 more gold in upkeep you get +1 reach and -0.2 weight (which actually is a bad thing considering weapon stun effects).
Now, let's compare 2 other weapons:
Persian War Axe
weapon length: 90
weight: 4.5
difficulty: 16
speed rating: 91
weapon length: 90
thrust damage: 0 pierce
swing damage: 42 cut
slots: 2
Unbalanced
Bonus against Shield
4035 gold
Bardiche
weapon length: 102
weight: 3
difficulty: 14
speed rating: 91
weapon length: 102
thrust damage: 0 pierce
swing damage: 43 cut
slots: 2
Unbalanced
Bonus against Shield
4876 gold
In this case you get +1 damage and +12 reach, with -1.5 weight for a mere 800 gold more.
Why does the +1 reach on a longsword vs heavy bastard cost so much more compared to the gold cost for +12 reach on the bardiche compared with the persian axe?