Proportionate Share?

I was listening to an audio version of Exodus in the car this weekend.  I tend to listen in chunks instead of doing the smaller, daily readings, which is detrimental to in depth discernment, but that's nonetheless what I was doing.  So I'm not sure if we have officially reached this point in the scheduled reading or not, and I can't pinpoint chapter and verse.  But what I heard was the long recitation (twice) of all the rules and regulations of Hebrew life, and one of them had to do with every person giving a fractional portion of a piece of silver as part of an annual offering of atonement.  The instructions were very clear:  the same amount was due from every man; the rich could not be required to give more, nor were the poor entitled to give less.  I'm probably revealing an unacceptable socialist bent here, but it doesn't seem fair to me that the poor would be required to give such a significantly greater portion of their assets than the rich.  Or, looking at it another way, if great sacrifice was the aim of this measure, it doesn't seem fair that the wealthy would be deprived of the blessing of that spiritual discipline by restricting the amount of their offering.  Why not a percentage instead of a "flat tax?"

LB