I see the 502 for $29.00, but the 505 is listed for $99.00.
The difference btwn the two is that the 502 is a two poise beam while the 505 is a 3 poise beam. Also the 505 has a whopping 6 grains more weighing capacity. I think they'll perform almost identically for most loading issues, so I'd go for the 502 if you need a scale. $30.00 beats $100.00 for a back-up scale.