Be ready to walk

Say, kindly, that it is more than you wanted to spend and you will keep looking. Mean it.

When it works

When the thing has been for sale a while, or you really do have other places to look.

What to say

“It's more than I wanted to spend, so I'll keep looking. Thank you for your time.”

The mistake to avoid

Threats and ultimatums. Calm and leaving works; loud and staying does not.