Quality parts are going to cost you more,
The amount of money that you spend should be based upon the type of games that you play.
If you are doing Backyard AS, or small Friend Speed games, then most tuning and tuning parts are really not for you, yes it is great to have the best AS Gun on the playing field, but sooner or later your friends are going to get sick and tired of getting Smoked by you and won´t want to play.
If you are doing small Events, and Team Games, then you should consider investing in some tuning parts, just to keep you on a competitive level with the rest of the players. Later when you start taking part in BIG Mil-Sim/RPG Games and Events, there is no way of getting around investing some serious cash in you AS Gun.
As far as the different companies go, they all have Good parts, and Bad. I personally don´t recommend spending the money on `` Drop In ´´ GB´s, as a rule they are overpriced, and only deliver a good performance for a limited time( usually very short ). I have actually come to the conclusion that Companies intentionally put sub-standard parts in their complete GB´s, examples CA, substandard Pistons, SYSTEMA, Springs to include all the minor springs - under rated gears for Main Spring Power.
Compromises can be made in all levels of tuning to save money, it´s just a matter of knowing exactly what it is that you want to achieve.