Who should pay lock extension fees?

Typically no but you can certainly ask. If you have an option to walk away you are in a stronger position.

The last couple deals I have had where the lock needed to be extended due to an avoidable seller delay, the seller paid the fee.

Dan Gotlieb
Digs Realty Group
www.digsrealtynyc.com

Many times my lenders will be open to waiving some or all of the fee depending on how many days are needed for an extension.
There are other alternatives also; just one of the many advantages of working with a mortgage broker.

Ellen Silverman
Licensed Mortgage Broker since 1990, NMLS#60631
Licensed Real Estate Broker

I would think it depends how far you deviate from the "on or about" date on your contract. The buyer/seller has some wiggle room (at least 30 days) from the expected closing date.