What I'm looking for is a pack that is similar to a haversack but provides for a familiar
Side pockets (2) ea hold 20 lb (per a handy haversack)
Central pack is an extra-dimensional 2' cu area that comfortably stores familiar, padded bedding, complete cover, and while closed it is water & air tight. While closed, movement and angle of pack does not change orientation within the packs extra-dimensional space (if caster upside-down, familiar is comfy so long as the pack is not open. Can be opened from the inside. It can store 5 lb of gear - typically materials for the familiar. Pack must be open for the caster to benefit from contact with his familiar.
So - what do you think the cost should be? I was leaning towards $2500
My faulty logic:
* much lower capacity then the haversack (45 lb vs 120 lb)
* lacks the reduced time to pull items out
* lacks auto contact provided by spell
* lacks free action teleport of familiar provided by spell
Well, what do you think, is it reasonable priced?
I think you may be overcomplicating the matter; it seems to me that a regular handy haversack could readily serve as a familiar carrier without modification. Extradimensional spaces, such as a bag of holding, have listed air limitations so I'd venture to say that they're already water and airtight once closed; otherwise there'd be no way to suffocate. As for the comfort and angle issue, there's nothing to say that such is not already the case for extradimensional spaces; there are no rules that govern damaging contents by violently jostling the bag/haversack so simply assume that whatever is inside is relatively secure no matter what goes on outside. Likewise, I see nothing in a bag of holding or handy haversack's description that says that they can't be opened from the inside; portable holes certainly can so why not other extradimensional containers? And, if the haversack is kept open, then there's no problem in having the familiar reach out to touch the master or vice versa. Just keep it simple...
I've got to agree with Ambrus, and really the tiny familiars could comfortably hang in either of the side pockets.
Honestly, this is just a complication caused by WotC not releasing the Familiar Pocket spell as part of the SRD. The haversack is basically a handwave item for "don't worry about encumberance" and all you want is a "don't worry about your familiar dying" item.
I'd say this is a common unadvertised use of the handy haversack and leave it at that. Use one sidepocket for your familiar, another for your spell components, and the big main compartment for your gear. Lots of worries saved.
Hrm. Ok, what if I add the bells and whistles?
* full capacity 120 lb
* reduced time to pull items out
* auto contact provided by spell, even while it is inside
* free action teleport of familiar provided by spell
Now how much would you pay? Wait, we will double the order, just seperate shipping & handling...