New magic item

All the recent discussion on the list about long range spells and
devices has given me an idea for a new magic item.

Spellcannon / Magecannon / Magegun

Appearance.
This device is a six foot long rod of crystal mounted so that it can
be moved to face any direction. It glows softly with an innerlight  in
the presence of a active spell jamming helm.

Mechanics.
The device is able to increase the rang of any spell cast through it
to 4 Hexes but it costs the ship SR dependant on the level of the
spell cast through it.

I have two ideas for how to calculate the SR cost.

Idea 1
The ship loses 1 point of SR for a number of rounds equal to the spell
level.

Idea 2
The ship loses 1 point of SR per spell level, with the loss
diminishing by 1 point per round.

Naturally the losses from either method of calculation would be
cumulative.

The device has a THAC0 of 15?

XP value 4,500?
GP value 45,000?

(the XP and GP values are more likely to be adjusted down than up as I
feel that this item is less powerful than an accelerator)


As always I would appreciate any feed back from the list.