No it is not simony.
Offerings are in thanksgiving for the sacrament and the support of the Church. Canon Law governs the maximum offering for a sacrament locally through the bishops:
Can.* 848 The minister is to seek nothing for the administration of the sacraments beyond the offerings defined by competent authority, always taking care that the needy are not deprived of the assistance of the sacraments because of poverty.
Can. 1264 Unless the law has provided otherwise, it is for a meeting of the bishops of a province:
1/ to fix the fees for acts of executive power granting a favor or for the execution of rescripts of the Apostolic See, to be approved by the Apostolic See itself;
2/ to set a limit on the offerings on the occasion of the administration of sacraments and sacramentals.
The competent authority in this case is the bishops of the Province, or in absence of that the diocesan bishop. So, the offering for sacraments should be uniform in the diocese. You can call and ask for the documenation from the diocese, ours is online at the diocesan website in our diocesan finance manual.
In our diocese, the bishop has designated that offerings for sacraments go to the Works of Charity, not to the priest. The faithful are free to give a separate offering to the priest if they would like. In other dioceses, the bishop has determined the offering goes to the priest. That is his prerogative. In our diocese the suggested donation is $10 for baptism, $75 for marriage, and $50 for funerals. This does not include any fees for classes and materials.
As well as the offering given for a Mass intention: