Cheapest way to measure arousal levels
I think you may have answered your own question.

You can build a GSR meter or purchase one online. You do not necessarily need to hook it up to your computer; if it has an LED you can simply record the results with pen and paper. If you want to measure heart rate, buy a stethoscope. Or if you want to be really cheap, check your pulse using two fingers; it's free.

Note that I have no experience with building my own GSR meter or using the GSR meter that I've linked to. I cannot vouch for the accuracy or reliability of these devices. The familiar adage is probably appropriate: you get what you pay for.