It’s a twofer!
Post “which grading company do I go with” here: www.elitefourum.com/t/the-giant-professional-grading-thread/30930/1
Post “what’s the difference in value between graded cards” (although your question as you’ve stated it is impossible to answer without specific cards/grades, there isn’t a blanket 20% discount on all CGC cards) here: www.elitefourum.com/t/the-giant-english-market-thread/27424/1