If you do the math, Concord is actually right.

You see, 500 gold is $2.50 right? 1000 gold is $5.00, and the next, 1500 gold is $8.00 but it should be $7.50 because $5.00 + $2.50 = $7.50.

This is true, that's why last time I bought gold, I just kept using the first option. I saved myself some money with that lol.