Slightly poorly worded this one. It shouldn't say earn, since you're given 750 at the start and need to spend 750 to unlock this achievement:
Then earn and spend another 1500 to unlock this one:
You'll then only need to earn another 750 and buy another monster to unlock this achievement. Thus you'll have only actually earned 2250 and spent that, not the 2500 is says you need to earn and spend.