From my understanding, the offering for Mass intentions covers the necessities for the Mass - the host, wine, the priest's time, the heat for the church, etc.  An offering is asked (and probably expected) just as if you would give to a priest for a wedding, funeral, etc.  I don't think a priest would refuse a Mass for your intention if you can't afford it but for those of moderate means and up, an intention offering would be expected.  And no, it would not be considered simony any more than an offering given for a wedding or a funeral.