What impacts the cost of switching regulator
And why wouldn't you go with the cheapest part that meets all the requirements. If you think there's a reason, it's probably because you missed a requirement. Availability, for example. 

Don't forget to include the external parts, some older regulators are very cheap but operate at lower frequency and/or have low efficiency (eg. MC34063). Some have been cloned or second sourced and are inexpensive for that reason. Others are used in huge volume and are inexpensive compared to "boutique" parts aimed at niche markets.



As far as your followup question about using the same part number in multiple places, that's usually a good thing, however be careful you have allowed enough margin on, say, the input voltage. If your circuit is to be used in an automotive application, your requirements for handling input transients will be quite severe, and protective devices only take you so far. 

If you're working with a 12V regulated wall wart, you can probably be okay with an 18V rated device.