There shall be placed in the pension reserve all appropriations made by the legislature for the purpose of paying pensions and survivors' benefits and of establishing and maintaining an actuarial reserve and all gifts and bequests to the pension reserve, and contributions of persons entering the retirement system who have established prior service credit. Members establishing prior service credit shall contribute to the pension reserve as follows:
For the first ten years of prior service fifteen dollars per year;
For the second ten years of prior service thirty dollars per year;
For the third ten years of prior service forty-five dollars per year.