...er.... Bentonite IS clay! (Just a slightly different version of the many, many
available). I was only estimating based on the very small quantities that I tend to use in a bucket and, as I say, I haven't really done any accurate survey on it. If you're happy with your calculations then go with those.
Just using the Potclays site again, it looks as though
Bentonite is still going to cost you about £200 per quarter tonne in non-powdered bulk form. If we assume that 250Kg per 400 gallons will give you a thick enough mix, that would equate to about a quarter of one of those one-tonne drop bags that the builders' merchants use. It could be ok I suppose. I guess the trick would be to slowly fill up with water and keep mixing in the clay so that you haven't added all the water or all the clay first so you can still adjust if you need to but it's going to take ages to mix. If you're ok with handling bulk non-powder then the cheapest 'plastic' clay seems to be
ESVA ball clay at just under half that price.
Probably the best bet is the cheapest bulk powder they appear to do,
China Clay Powder (which should be nice and white anyway!) at about £116 per quarter-tonne, which is a lot cheaper than the Bentonite, actually, and still in powder form which should make for easier mixing. As you say, maybe order a 10Kg sack of that as a test (£6.74) and do some accurate water measurements to work out how much you'll need for the consistency you want.
I'd be interested to hear how you get on. I've not tried China Clay but if you think it works ok and if I can get that in powdered form from my local art supplier then I'll give that a go next time I need to buy some in. (Mixing the modelling clay from solid block form is pretty laborious!)
By the way, beware of the models ingesting any Bentonite if you use that - it's sometimes used as a laxative! Conversely, China Clay (Kaolin) is used in anti-diarrhoea remedy so might result in constipation (although I think the majority of that product relies on the corresponding use of morphine that is mixed with it)!