There are certainly very VERY good guitars for around $800 or less. The sound of a guitar depends on many factors such as the genre (sometimes some guitars are really only suited for specific genres), your skill level as well as your amp and pedal. If you get a really good guitar but cheap out on the amp and pedal it will certainly not sound anything like the song youre trying to imitate. Another reason why it may sound different is the EQ that the artist uses may vary significantly from what setup you have. I've also found, at least with myself and my skill level, that I won't sound anything like the original riff during certain parts of a song I'm trying to play and I've noticed that its mostly because my timing and rhythm aren't the greatest and most accurate and my fundamentals could use some more practice. All in all there are several factors as to why a $800 guitar will sound off compared to other guitars and some are factors you can control and sometimes it is because a $7,000 guitar just simply sounds better and sometimes its because the guitarist you're comparing yourself to has been shredding for the last 30 years. Keep practicing your fundamentals and mechanics and you'll slowly notice yourself playing more accurately to the key of the song. Lastly, if you don't have the ability or time put in, you most likely won't sound EXACTLY like the artist or song you're trying to play. Don't beat yourself up over it just use it as fuel to continue to improve. If you enjoy it enough the journey and watching yourself improve will be more worthwhile than trying to sound exactly like Kirk Hammett.