Subject Database design. Simple question!

Hello! I  have a FB 2.5 database with a tabla "invoices". I need to add a field in that table which will be null most of the times. Lefts say, that 90% of the records will have this new field in null. 

What is your advice? To create a new table with a FK to "invoices" and save the needed field in a separate table (only when the value is not null of course) of just create the new field in the main "invoices" table?

I think I read somewhere that null fields use very little space in the database. Is that true?

Thanks for any input!!!