In addition to what mpesce said, In general I rarely would do anything "free" for a large corporation, unless I saw how it could lead to work. e.g. if I needed a foot in the door or they could lend credibility to me. That doesn't mean I don't want to give back (I strongly believe in it and do it), it just means that large corporations have a history of taking advantage of smaller teams, individuals etc, yet they are the ones in the best position to pay fairly for quality. If you would feel better about doing it if paid then you can also try and offer to schedule it when it might be more conducive for you to donate your time, in which case they may then offer to pay expenses etc.
Just an alternative view too. This solely depends on your goals and whether you need this but you may consider doing the talk if this company can help raise your profile or their credibility might help you with your startup. In that sense you can view it as marketing or research that directly helps your startup. In fact, if the people you would be speaking to would be potential customers for your startup, I'd really think strongly about how you could do it and meet their talk but also do a little promotion at the same time.