This fantastic BBC article outlines the costs of elections, polling and referendums in the UK.
Notably, this gives you supporting evidence for the impact of referendums and the challenge they can produce to parliamentary sovereignty – if referenda cost this much to stage, would the government then simply ignore the outcome to follow their own beliefs? The likelihood of this is limited, as it would lead to questions over why so much money was spent consulting the public just to ignore them. This could arguably therefore make referenda de facto binding on the UK Government (while remaining, de jure, not legally binding). This could be seen as a threat to Parliamentary sovereignty.