Certain sectors of the New York City policy world (the Municipal Art Society, the New York Times editorial board) have been calling for a while for the relocation of Madison Square Garden, so that a new, grand Penn Station could be built in its place. (The old, grand Penn Station was demolished in the 1960s to make way for the current Madison Square Garden, the fourth building to bear that name.) NYU’s Rudin Center for Transportation Policy and Management released a study last week of how much it would cost to do this, and came up with … do I hear $5 billion?

This isn’t really all that surprising: A billion and a half for a new MSG sounds about right given that just renovating the old one cost a billion, and acquiring new land could easily cost half that in this market. (The Rudin report looks at the price of buying up the annex to the Farley post office building across the street Morgan post office annex a couple of blocks to the southwest, but other sites would be priced similarly, if you could even find any.) And almost $3 billion for building a new Penn Station is already the price tag established by Gov. Andrew Cuomo for his plans (which would leave MSG intact but build lots of new stuff under it).

It’s also important to consider the political context, with Cuomo’s plan to expand Penn Station with MSG in place (to be paid for by some as-yet-unidentified private developer — applications were due two weeks ago, but if any have been revealed it’s news to me) going up against the MAS and Regional Plan Association’s insistence that MSG really needs to be kicked out. Given that Rudin director Mitchell Moss has already endorsed Cuomo’s plan, and his report’s conclusion is “It’s time to move on,” it’s easy to see some political gamesmanship going on here.

Still, this whole mess is a reminder that as easy as it is to envision redesigning your city to undo past mistakes (tearing down one of the greatest public spaces ever, building a kind-of-ugly sports arena in its place), there’s something to be said for actually existing architecture, both in that it’s already paid for, and in that the city has grown up around it to accommodate it. Not to say that nothing should ever get built or torn down, but it’s important to look at the true costs of doing so, and whether the money could be better spent mitigating the effects of your last mistakes.