The least expensive battery packs (includes DC cable, carrying case) I have seen are on CPAPxchange, for half the price you mention. Have not used them myself. There is no smaller BiPAP than the S9 (I would decouple it from the humidifier) that I know of.
We use a Z1 CPAP w/ battery pack for going out and sling transfers. We were out yesterday for ~4 hrs and it went down 2 bars of 4, so that is similar longevity. We are in drizzly Seattle and the machine can rest on my husband's chest, underneath his blanket/poncho, so it is protected from the rain. I am not saying that this would not apply to a carrying case on the wheelchair back, but I would probably have more concerns about water and jolts.
In the wheelchair or sling, we loop the built-in lanyard on the sling cradle or wheelchair control. It uses slimline tubing. We got the Z1 as our "portable" because it hangs vertically or sits horizontally and the S9 (which we still use at night) is not designed for vertical use. We do not have room to roll the S9 around on a cart or table but if you do, that may mitigate the Z1 advantage of vertical positioning. We keep an old ST in the living room for when he is there.
Of course, the #1 disadvantage of this scheme is that the Z1 is a CPAP. In its more aggressive mode of expiratory relief (smoothing the curve rather than EPAP, of course), my husband can live with it for a few hrs but it's not as comfy as the BiPAP. Reportedly, a more aggressive mode is forthcoming (firmware is upgradable on line) and we will definitely dial that one in.
