Those part are expensive because of their package. The ceramic materials, gold bond wires, gold plating, hybrid thin/thick film technology adds today a lot to the final "market price" (95% my guess). The reference itself could be kind of a "12-14bit ADC/DAC" parameters (at typical 10V FS it means something like 600uV resolution), however (as the most references we talk here were designed long time back for that purpose). This APEX ref is also expensive because of its trimmed initial accuracy, so good for a mass production of goods required such an initial accuracy. What people want in this section is the long term stability, low noise, low TC and the initial accuracy is not so important.