"vanity sizing" is a term consumers use to describe size inflation.
The numbers mean nothing, they are arbitrary. Numbers used to mean something but that's another long story (http://www.fashion-incubator.com/archive/the_birth_of_size_10/).
Point is, a size 10 is the size that manufacturers use as the midpoint of their customer base. It is their "average" customer. Now since some lines target thinner women, their size 10 will be smaller than the size 10 of a line that targets a heavier customer. It is unreasonable to expect size 10s from various lines to be similar. The size 10s of more expensive garments (Ann Taylor) are smaller than the average size 10 who shops at Old Navy.
If you expect consistency in sizing, buy from the same brand. Of course it doesn't make sense to do that in real life. We all buy clothes from a variety of makers at different prices and for different needs. If I want a tee shirt to muck out the barn, I'm sure as heck not going to buy a $50 tee shirt from Ann Taylor, I'm going to Wal-Mart to pay $4. The thing is, the average Ann Taylor customer is much thinner than the average wal-mart customer -so why should Wal-mart size their tees for my rare purchase there just because I'm too cheap to fork out from my usual brand?
The reason people notice the changes now (it has always existed) is because people have the disposable income to buy from brands (and so, be able to compare) they could never consider buying before. Clothes are cheaper than ever, prices have not kept pace with inflation. Even in strictly dollar amounts, you can buy clothes at prices never before possible. It was impossible to buy a new tee shirt of a given quality even ten years ago. At Wal-mart, I notice the $4 tees are now selling at $3.50.
Depends on the size and where you get it done. Letters numbers usually run from 1-4 dollars per letter or number. Full piece sets usually run 10-20 just an estimate. Hope this helps :). Buying in bulk I'm not to sure about sorry.