That's what I meant with "high price". Not literally an "exit fee" (which is also on the table btw) but rather a set of economical, financial and political consequences. For instance, the inability to renegotiate trade deals as advantageous to the UK as they are now because the EU doesn't have any interest in sweet talking the Brits into joining the union this time, so it's likely they won't get the same favorable treatment they received in the past. Same thing happened with Switzerland: at first the EU was eager to comply with Swiss requests for special treatment but as the Union grew stronger and the Swiss electorate turned down the first deal (to join the EEA), Switzerland lost part of its negotiating power and had to accept less favorable conditions.