Price on any good has always depended largely on how much of the good is already on the market verses much the customer wants the good. Dwarves are opposed to slavery so the value of a given slave on the Dwarven economy is negligible since they're just going to end up setting them free. The value is further decreased by the Dwarves' typically having good trade relations with Human empires, since Humans see slavery as acceptable and can provide their friends with a constant supply of a good they don't even want.
Culturally speaking, the reasons for their opinions on slavery probably has a lot to do with the labor types they require. Humans live above-ground with gigantic castles and tombs, if you've ever tried to build above-ground exclusively, you'd know that it takes having a lot of unskilled workers just to cut blocks and construct the outer defensive wall. Dwarves, by contrast live underground, they only need enough workers to dig out the negative space to be inhabited, and so most of their population ends up in various forms of crafting and soldiering.
In short, you're bound to get a low price for sapients as long as one of the negotiating merchants is Dwarven, because they're not going to see the value of forced labor and base the price entirely on the cage. (And it's not like selling the cage to Humans will realistically change that, your Dwarf already marked the slave as next to worthless, they'd be fools to correct him and have to shell out more cash for their killer deal.)