general rule of thumb -
*loose common G3's $2-3 (core 7, first year or two)
*More sought after ones $5-7 (cutie cascades, 3D symbols, Unis, Pegs)
*playset ponies will vary on what playset they were released with and if they were re-released at all outside of that playset
*Holiday releases $5-15 depending on which ones
*exclusives and special releases will vary as well