That precisely describes the example I gave (sales tax that funds pointless art projects noone likes), except the cost to the economy, and the impact on the poor, is readily quantifiable, unlike hypotheses of various opportunity costs regarding parking.

Saying all parking is funded the same way and has the same externalities is both reductionist and absurd. Furthermore, what value empty lots provide is entirely up to the people who control them; several around here are used for farmers markets and other activities outside of peak time usage.

We've got light rail and buses where I live; one of the light rail lines was poorly planned and is now subsidised somewhere around $20-30 per passenger per trip. Another was promised to ease traffic congestion and revitalize several neighborhoods along the path; it has done neither and has arguably made both worse (traffic has to stop when it goes by, and the horns contribute significant noise pollution) and driven down property values to a certain degree.

Basically, costs are relative, and arguing over parking is far less productive than what we could be working on in our communities.