[dpark]

Probably $500 or so if you need a breakout panel installed. Unless you need a full panel replacement or a service upgrade, getting a spot for the breaker probably isn't prohibitively expensive.

[secabeen]

Yeah. In our area, 50-year old panels are often discovered to be Zinsco-brand, and should be replaced for safety reasons.

[dpark]

In which case you should budget for that regardless of the new 240 line, of course. :)