一定去!

听来,你工作的领域不是那种到处都有,位置俯拾皆是的行当.为了成长的potential,不应在乎小的花销.

其实,即便这次你不能拿到位置,只要你的presentation成功,我都认为是合算的-你没有损失一次让别人认识你的机会.Who knows, maybe the one they hire turns out not a good one?  Maybe someone in the audience introduces you to other opportunities? Worst case you lose a day or two and a few hundred dollars, but compared to a possible better position?

Most of all, you do not know what their intention is - whether they already counted you out, or they want to see your breadth of knowledge, or something else? If you can find a position easily now and then, here and there, you wouldn't care.  But if that's not the case, you do.

AND, I do think your presentation is not that difficult. You have learned a lot in college, in grad school. Pick one of the topic you feel comfortable, there may not be many people there in your audience that know much about it. (Make sure you do not talk about anything those people deal with day in and day out.)  And still, you can open with "per the request of my host, I am presenting a topic that is not my expertise. If you know this topic better, I consider this presentation a chance for me to show my ANALYSIS capability in answering your quizing questions". That's the best part of this - even if your presentation were awful, it would not hurt your reputation.

Good luck!