My yearlings are all over the board. I have two smaller ones that are the same age as the others that are about 600 grams, and then I have several others that are over 1000 grams. It all depends on the snake. The smaller ones are kept in the same rack, so same conditions and are fed the same thing. The smaller ones are more likely to miss a week and are a bit more picky eaters.
I think some of it may also have to do with their locale (where they or their parents/grandparents) were collected.
i got my scale from walmart for $30. it does grams and pounds, and is very simple to use. that hardest part is getting the lil corn snakes to stay still on it to get a reading.
I have that scale (the 7001DX) and really like it a lot. The hold function is perfect for keeping the weight showing while you tuck the wiggling snake back in it's home. We don't have the AC adapter but it doesn't seem to burn through batteries. I weigh everything from tiny 07's or the milksnake or gecko, all the way up to the boas and the big BP's on it.