The deeper they go in a line, the more EP they give the character, so they cost more.
So Emma's(and Elliot's?) Arcus, being one single line, has the most expensive Orbment slots.
Honestly it doesn't make much sense with the Orbment system in Cold Steel. In Trails in the Sky line length also restricted the higher level arts you could use, as they required a combination of Quartz all in a single line, which added value to unlocking slots in one line.
But in Cold Steel the system has been balanced out to benefit non-casters as well. (There are now benefits to Physical attackers having multiple lines) and the line system for arts has been removed, so I'm not sure why the cost has to be higher anymore