Why are these similarly spec'd DC-DC converters an order of magnitude different in price?
Operational temperature range is always major factor on why power supplies cost extra.  I can't find any temperature range for the cheap supplies, but the expensive one you listed has an operational range from -40\$^\circ\$C to 80\$^\circ\$C.  That is a pretty robust range to meet the datasheet specifications over and that extra robustness is translated to you in the form of cost.

Other factors that usually drive up the cost of a power supply are (by no means a comprehensive list):


Input Ripple Rejection - The effect that noise on the input of the DC-DC converter has on the output.  More noise filtering will cost more money.
Output Ripple - The amount of steady state voltage ripple (generated by the switching) on the output.  Less ripple = more money.
Efficiency - A measure of the % of wasted power (turned into heat) during the conversion from one voltage to another.  Higher efficiency = more money.
Synchronicity - The necessary diodes (to isolate the direction which current can flow) are replaced by tightly-controlled, switched FETs so that you don't have to deal with a diode voltage drop.  This greatly increases efficiency (&amp; complexity for the manufacturer).  Synchronous DC-DC converters usually cost more.
Monolithic - How much of the necessary supporting components for the DC-DC are integrated into the module itself. Usually the more the DC-DC does internally, the more expensive it is.
Transient/Load Step Response - How fast can the DC-DC deal with huge changes in demand for current &amp; what change will that have on the output voltage.  A faster response and less change in the output voltage under large changes in demand for current from the DC-DC will generally cost more $.
Pulse Skipping - At light loads the DC-DC converter can "skip" conversion cycles if the output voltage has not decreased substantially.  This greatly increases efficiency and usually adds complexity for the DC-DC manufacturer, which is generally translated to you as 
higher cost.
Allowable Mechanical Stress - How much mechanical force can the module withstand without breaking &amp; still reliably functioning.  The cost of the time to test this and the components that can withstand higher stresses are translated to you as higher cost.
Certification (UL/CE) - The standardization process takes time and money to complete and that cost is recouped by charging you more money for the product.