Q:

About voltage divider resistor network

I need a precision decade voltage divider.  And there is a product from Visahy just can do what I want.  But it's soooooooooo expensive!  So

Why they are so expensive?  
I need the ratio tolerance at least ±0.05%.  And if it's acceptable the absolute ratios may not be in 'decades' precisely.  Can I use several discrete 0.1% metal film resistors in series to achieve this?  

A:

You need to do two things to build your own precision divider, unfortunately you will need to spend some money.  
a) adjust resistors initially to get the ratio
b) use resistors with good stability, so you keep your ratio
Adjust resistors
It is much easier to match resistors precisely, than to set them.  It allows you to use a low-precision DVM to measure the voltage from a Wheatstone bridge to high accuracy.  
If you want to make a decade divider and want all the taps, put 10 matched resistors in series.  If you need only one tap, put 3 in series, 3 in parallel, and then you have a 9:1 ratio pair of resistors. 
Keep their precision
As you spend more money on your resistors, you can get tempcos improved from the standard 200ppm/C to sub 20ppm/C, and the same goes for long-term stability.  It can also be worth buying multiple matched resistors in a single package, as often their tracking performance is specified to be better than you could acheive with discrete resistors, because they share temperature, strain, impurities etc
Of course, you could use standard '10 a penny' resistors, and re-check their values before each use.