The first thing to be aware of is that although many operators advertise Fixed Departure Treks, the reality is that unless sufficient trekkers turn up then the trek doesn’t run
So, partly for this reason there is absolutely no point in trying to book up in advance
You would be far better off reading the partner adds on sites like trekkingpartners and putting your own trek together.
If you found one other trek-mate you could just hire out one porter/guide and he will both guide you as well as carry 15k, so this saves you packing most of your gear, costs are around $20 per day including his food and accommodation as well as his insurance but not transportation to and from the trail-head or tip + you will need to pay around $25 /$30 each for your own food and accommodation, $30 each ANCAP and $10 each TIMS ($20 if you organise this in advance through an agent) – This will work out a Lot cheaper than joining a group + you can walk at your own pace, stay where you want and eat where, what and when you want.
You will meet lots of other trekkers in the lodges in the evenings so you don’t have to worry about the social aspect of the trek either ;-)