You make music with your ears, so treat them well and get some proper monitors/headphones. seriously. You can make good music with cheap equipment, if you can make it sound good on proper reference devices.

I for one also like those AKG 240s very much, and they aren’t far off to your price range. They lack a bit transparency in the lower end but you have a very good hi/midrange response and that’s one of the most important parts when it comes to monitoring.

But the most important advice always has been: train your ears! When getting better headphones your ears, or more precisely your brain will have to adjust to the loads of new information it has to process. Also use those headphones to listen to your favorite music as much as possible. That way you’ll learn a lot about sound design, mixing and the depth of putting your stuff together as you really want it sound.