considering you told us nothing about which currency you wish to measure, i will assume australian, if so you only need 1g accuracy to diffentiate every coin, still the second digit wouldnt hurt,
i would definatly reccomend just breaking into the 7 segment display, you have 7 wires that will drive the segments, and 1 transistor per 7-seg to pull down its earth, tie as many of the segment pull downs as you want to interrupt lines, and have your micro read the values of the 7 segments each time an interupt is triggered, and store them into a array for each segment, that means only 8 pins for 1 digit, 9 for 2, and so on,
also to me this seems a little tidier than the other 2 solutions :/
edit: if you want the dot point that will cost you an extra pin,