Is it o.k. to ask temporary employees to contribute to parties, etc.?

I'm in the same boat as you. However, if you do not contribute, you shouldn't attend. If you place yourself on the outside, thats how they will always see you. If you want to be part of the group (and possibly have the opporotunity to get hired) you should participate in things. Maybe not every single event, but you should do what you can. I make half of what everyone else at my company is making. It sucks, but I do not alienate myself. You are not a guest. You are there to perform a service, just like everyone else. You should not be treated differently (which I think should include pay), but that's the life of a temporary employee.

right now the regular employees where i work as a temp are planning a halloween party, the rules are, if you wanna eat bring something. utensils, bag of chips whatever you can afford... and if you participate dress up (the theme is wizard of oz) they asked me, i told them a simple no thanks... no explanation necessary, i know i cannot afford food, and i know i do not want and cannot afford to dress up this year, but no one has shunned me and no one thought anything of it, and i know i won't be eating their food... it is that simple

No, I don't think you are right to feel this way. You shouldn't expect other people to 'pick up the check' just because you make less money than them. How are you even sure you make less than them anyway? Maybe you are not an employee of the company, but you still work there. They are just trying to include you and make you feel like part of the group.

I'd like to add to rtfm's excellent answer... You should actively participate in the company's social life because it allows you to build and expand your network. As a temp, this is very important and well worth the money and time. And do count yourself lucky. Lots of temps find they are not even considered human in many company settings.