I have heard and read several times that one of the forces that drive language change is the so called "principle of least effort". According to this account, several changes are caused by an economy in pronunciation. Take this Wikipedia explanation, for example:
Speakers especially use economy in their articulation, which tends to result in phonetic reduction of speech forms. See vowel reduction, cluster reduction, lenition, and elision. After some time a change may become widely accepted (it becomes a regular sound change) and may end up treated as a standard. For instance: going to [ˈɡoʊ.ɪntʊ] → gonna [ˈɡʌnə], with examples of both vowel reduction [ʊ] → [ə] and elision [nt] → [n], [oʊ.ɪ] → [ʌ].
Is this a scientifically valid assertion? If so, how do you measure the cost (and thus the economy) of the pronunciation of a certain sound in a language?