Punked by an empty promise?

My boss has promised me to give me minimum $10,000 US dollars bonus after the project I am working on is completed. However, my Christmas bonus this year is just half of the amount I got last year, and the reason according to my boss is that the company does not make any money this year, so I wonder how I can be sure my boss will not use the same excuse not to give me the bonus or cut my bonus to half after the project is completed?! It just does not make any sense, can anyone give me a good advice whether I should stay and work on the project till the end and hope my boss will not screw me over or I should just quit now and take another job that offers better salary?