Vintage video game value guide?

So I've been looking on ebay at old vintage NES, SNES, Playstation, etc games that I had owned and played as a child. I noticed that many of these games sealed hold a high price tag, such as SNES Chrono Trigger sealed is very valuable. I'm interested in collecting and perhaps getting some the games graded like PSA does for collectible cards. I would focus more on originally sealed games and don't want to pay to much for them. Is there a value guide available for these games? So I can check the prices before my purchase?