As far as I know, the only one below $500 is the Brother PE500. It only has a 4 x 4 hoop and can only use designs from your computer if it's connected via USB. Other than those two limitations it's a very popular machine with great reviews. There's also the Brother PE770 which is about $600 and has a 5 x 7 hoop and can use a USB stick.
Do keep in mind that once you buy a machine, if you want to make your own designs you'll need to buy software, which costs $$ as well. The machine comes with a lot of designs but there are more online that you can buy or find for free. If you never want to make your own designs, and the ones you buy or find for free are the correct file format for the machine, you shouldn't need extra software.