I did a search on eBay and yes, found some separate heads - but their prices are so high, about 118 USD, while even Tony Stark's and Volverine's heads are about 60 USD or less 0________o
Well, I don't sell the bodies, and it greatly depends on the body brand. 300-500 USD for the body, could I say. Plus head, I gave you the link already - the pricing is there too.
BJDs are also huge, like 1:2,5 scale.