Avoid an answer that involves money, an exception is sales where a key motivating factor could be exceeding sales targets and getting commissions.
Analyse the job description for keywords and tell a story around it. For example the job description for a project manager vacancy may require experience dealing with suppliers.
A good answer could be as follows:
I am motivated by the recognition I get from my stakeholders whenever I successfully deliver. In my last position I was involved in the procurement process to select a supplier to develop a software application. The quotes were out of budget, but this only motivated me to persevere until I found a supplier in Bulgaria whom who met our requirements. The project was a success and my managing Director congratulated me on successfully brokering the deal.