From PCPartPicker, the cheapest i5 that can replace your G3258 is the i5-4460, for ~$180USD from (pcpartpicker, Intel Ark)
If you want to bump that up to a hyperthreaded quad core, the cheapest are the E3-1230 V3 / E3-1231 V3 for ~$240USD (pcpartpicker, Intel Ark)
Just for the record, the cheapest i7 in the same generation is about $300USD