I know, it's a ripoff
According to Table 7-33 in the DMG it goes like this:
Use-activated or continuous Spell Effect:
Base Price: Spell Level*caster level*2,000 gp
Now, it's a Harper Scout 3, Ranger 3, Sorcerer/Wizard 3 spell, so as to make it as cheap as possible, we'll do it as a wizard. Hence, it would be like this:
3*5*2,000=30,000 gp. Now, according to Table 7-33, you have to multiply this base price by a factor determined by the duration of the spell. Since
tremorsense lasts 10 min/level, the table says that you have to multiply the base price by 1.5, so it would be 30,000*1.5= 45,000 gp. To create the item, it would cost 22,500 gp, and 1800 XP, Ouch!!!

.
I find it rather expensive, since it's not
that powerful an item, but then again, there are others that are even more useless and are more expensive. Well, that's all.
Branok