Some researchers will have the funds to pay you a small stipend (usually less than NSERC) for your summer work. You may or may not also be able to apply for USRA (industrial NSERC--there's a similar thread here regarding that program)--the application for that is four weeks before the start of the job.
If you offer to work for free, yes, a prof will probably take you on. Now, the question is whether or not you want to do this, given that you could have a (non-research) summer job instead and get paid *something*. Whether or not the opportunity cost is worth it is up to you (or you could split the difference and work part / full time at both).
Good luck with your decision making.