Areivim is not like giving a dollar for an apple, because you are not necessarily getting the apple. first, you only get the apple if you die with unmarried kids under a certain age (and technically you are not getting the apple, your kids are), and second, there have to be enough apples left to give out.
A better mashal might be a Chinese Auction- you give money to the organization, that money will go to help others. There is some chance that you may gain something too, perhaps more than you gave. I think that it is muttar to give maaser to buy auction tickets (maybe because the chance of winning is low?)
