Often, when speakers start out in the business (and sometimes even after they are established), they will offer to do free events. But a free event does not necessarily have to devalue you as a speaker. In fact, it can give you a number of benefits, if you go about it the right way.
Always go in with a positive attitude
This free event may just be a way to get practice. It may be a way to gain exposure. Regardless, you need to approach it with the same professionalism as a paid gig. Your attitude will shine through your presentation, and if it is positive you could get more business from this free gig.
Make other value arrangements
You can make arrangements to (gently, without being sales-y) make an offer to the audience at the end of your presentation. This can be a freebie in exchange for their contact information. It can also be a gentle statement that you are available for hire is whatever capacity and provide a way for people to learn more.
You have to be careful with this one. Your presentation needs to be educational and not a sale pitch. And some venues will not let you share a way to contact you at all. But, if you can arrange this, your free gig has just turned into a lead-generation opportunity.