Why am I only gaining +8 ELO when im only 500 rated?

While playing blitz, I noticed that I only gain 8 elo and sometimes even 5 elo even thought Im 500 rated. Any help?

The amount of rating you gain or lose per game is not dependent on your rating, but rather how many total games you have played. In other words, the more games you play the more certain Chess.com is about your true rating, and therefore the less your rating changes. 

Your rating typically changes by +8 for a win, 0 for a draw, and -8 for a loss when you're paired against a player of similar rating to you. You will gain more points for winning and lose less points for losing when playing against players higher rated than you, and gain less points for winning and lose more points for losing when playing against players lower rated than you. Here's an article with more info:

https://support.chess.com/article/210-how-do-ratings-work-on-chess-com

The site uses the Glicko rating system that includes a measure of rating uncertainty called the rating deviation (RD) value. New accounts and accounts with few games in a pool have a high RD value and will have larger rating swings after games.

As you play more games the RD value drops as does the magnitude of rating changes.

When i buy premium does it stays at an level of like 50 elo?

No, premium membership doesn't change rating calculations.

if you are only gaining 8 elo, it takes less time than you think, trust me

rating changes have nothing to do with your current rating, only in relation to your opponent, who most likely has about the same rating as you do. hence the actual change in score will be small. likewise if you won against a player much higher than you, you'll earn a lot for that win and vice versa. if you lose to a much higher rated player you barely lose anything - since you're not expected to win that game