It's hard to do that.. but for the BEST sound for only $500 and you can actually build on it in the future is go on ebay and pick up a brand new samson drum mic pack. There is an ebay dealer who gives free shipping, and 7 free mic cables with it along with the 7 samson drum mics for about only $125. All brand new, and the mics are amazing. Headphones? You should just pick up a $30 pair of Audio Technica. I have
the ATHM20 ($50) and there incredible, and i got them new for only 25
mixer wise, my suggestion is go for an analog mixer because you get more out of your money. Get a $100-$150 behringer analog mixer that can hold up to 8 mics and your set.
Thats like $350 and you already have a mixer, drum mics, After that pick up an sm57 online for about $70 including shipping new. For monitors? I’d suggest anything around the $100 range like m-audio or something if you want to get something worth using. I have a pair of edirol monitors I started out with that only were $100 including shipping. It can hook up to 2 different computers at once and has settings for bass/treble/volume for both and pan.
If you decide to go with all that its $400. With the other $100 you can either pick up an at2020 condensor to use for vocals and even overhangs if needed. If not then you could pick up a blue tube 1 channel for the vocals r cymbals or what have you. I also heard the $60 art pre is really good, especially for the money.
Hope this helped. I personally think all this is a good beginning set up for $500. And everything can be upgraded or used later.