Hey Jack. I haven't ever kept black-headed pythons myself, but your signature line compelled me to post anyway...lol. The information that follows is purely anecdotal and based upon things I've read, observed, and been told by others.
"How much do they generally cost?"
I've seen babies for sale in the classifieds for anywhere from $2,000 to $3,500. I've seen proven adult pairs for as much as $12,000 for the pair. In other words, they are expensive, but IMHO, well worth it.
"How big do they get?"
BH's get larger than their woma cousins. Their average sizes seem to be between 6 and 10', with females being larger than males. I'd say 8' is a very easily reached adult length for a BH.
"What sort of temperament do they have?"
From everything I've heard, most are great. Both the Aspidites pythons are well-known for their feeding responces, so caution must be taken while they are inside the cage, but outside the cage, they are supposedly very docile animals.
"How easy are they to look after?"
People tend to refer to Aspidites as the "kingsnakes of the python world" in terms of their ease of care. They don't need any special requirements in the form of humidity or substrate, and they feed voraciously. BH babies may go a couple months without eating, as they tend to eat reptiles in the wild, but once started on rodents they don't stop. BH's are supposedly more difficult to breed than womas, hence their much higher price. Some people give them substrates to allow them to burrow, and I imagine they'd make use of this. Others give them newspaper and they still do great.
I hope this is of some help. I would have just waited and let someone else who is actually experienced with the species reply to you, but sometimes that doesn't happen and these posts make their way further and further down the page with no answers. Good luck!
-Chance
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Chance Duncan
http://www.rivervalleysnakes.com